Initial Feature Flag system

Why are these changes being introduced:

* FlipFlip has not been maintained in years and we believe it is causing
  more problems than it is solving. We need a simpler, more maintainable
  feature flag system.
* Our needs are simple: we need to be able to turn features on and off
  based on environment variables.

Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/USE-77

How does this address that need:

* Created a simple Feature model that reads from environment variables
  to determine if a feature is enabled or not.
* Introduces consistent naming patterns for feature ENV variables.

Document any side effects to this change:

* This only introduces the new feature flag system. No existing
  feature flags have been migrated yet and FlipFlip is still present.

Author: JPrevost

app/models/feature.rb

@@ -0,0 +1,49 @@
+# Central feature flag management class for the application. This class provides a simple,
+# centralized way to manage feature flags throughout the codebase.
+#
+# Feature flags are controlled through environment variables with the prefix 'FEATURE_'
+# followed by the uppercase feature name. Each flag defaults to false unless explicitly
+# enabled via environment variable to ensure consistency in how our features work.
+#
+# @example Basic Usage
+#   Feature.enabled?(:geodata)       # true if FEATURE_GEODATA=true in ENV
+#   Feature.enabled?(:unknown)       # Returns false for undefined features
+#
+# @example Setting Flags in Environment
+#   # In local ENV or Heroku config:
+#   FEATURE_GEODATA=true             # Enables the GDT feature
+#   FEATURE_BOOLEAN_PICKER=true      # Enables the boolean picker feature
+#
+#   # Any non-true value or not setting ENV does not enable the feature
+#   FEATURE_GEODATA=false            # Does not enable the GDT feature
+#   FEATURE_GEODATA=1                # Does not enable the GDT feature
+#   FEATURE_GEODATA=on               # Does not enable the GDT feature
+#
+# @example Usage in Different Contexts
+#   # In models/controllers:
+#   return unless Feature.enabled?(:geodata)
+#
+#   # In views:
+#   <% if Feature.enabled?(:geodata) %>
+#
+#   # In tests:
+#   ClimateControl.modify FEATURE_GEODATA: 'true' do
+#     assert Feature.enabled?(:geodata)
+#   end
+#
+class Feature
+  # List of all valid features in the application
+  VALID_FEATURES = %i[geodata boolean_picker].freeze
+
+  # Check if a feature is enabled by name
+  #
+  # @param feature_name [Symbol] The name of the feature to check
+  # @return [Boolean] true if the feature is enabled, false otherwise
+  # @example Check if a feature is enabled
+  #   Feature.enabled?(:geodata)
+  def self.enabled?(feature_name)
+    return false unless VALID_FEATURES.include?(feature_name)
+
+    ENV.fetch("FEATURE_#{feature_name.to_s.upcase}", false).to_s.downcase == 'true'
+  end
+end

test/models/feature_test.rb

@@ -0,0 +1,79 @@
+require 'test_helper'
+
+class FeatureTest < ActiveSupport::TestCase
+  test 'defined features default to false' do
+    refute Feature.enabled?(:geodata)
+    refute Feature.enabled?(:boolean_picker)
+  end
+
+  test 'undefined features return false' do
+    refute Feature.enabled?(:undefined_feature)
+  end
+
+  test 'features can be enabled via environment variables' do
+    ClimateControl.modify FEATURE_GEODATA: 'true' do
+      assert Feature.enabled?(:geodata)
+    end
+  end
+
+  test 'features can be disabled via environment variables' do
+    ClimateControl.modify FEATURE_GEODATA: 'false' do
+      refute Feature.enabled?(:geodata)
+    end
+  end
+
+  test 'environment variables are case insensitive' do
+    ClimateControl.modify FEATURE_GEODATA: 'TRUE' do
+      assert Feature.enabled?(:geodata)
+    end
+
+    ClimateControl.modify FEATURE_GEODATA: 'True' do
+      assert Feature.enabled?(:geodata)
+    end
+
+    ClimateControl.modify FEATURE_GEODATA: 'false' do
+      refute Feature.enabled?(:geodata)
+    end
+
+    ClimateControl.modify FEATURE_GEODATA: 'FALSE' do
+      refute Feature.enabled?(:geodata)
+    end
+  end
+
+  test 'non true boolean values default to false' do
+    ClimateControl.modify(
+      FEATURE_GEODATA: 'invalid'
+    ) do
+      refute Feature.enabled?(:geodata)
+    end
+
+    ClimateControl.modify(
+      FEATURE_GEODATA: '1'
+    ) do
+      refute Feature.enabled?(:geodata)
+    end
+
+    ClimateControl.modify(
+      FEATURE_GEODATA: 'yes'
+    ) do
+      refute Feature.enabled?(:geodata)
+    end
+  end
+
+  test 'feature names are case sensitive' do
+    ClimateControl.modify FEATURE_GEODATA: 'true' do
+      assert Feature.enabled?(:geodata)
+      refute Feature.enabled?(:GDT)
+    end
+  end
+
+  test 'multiple features can be controlled independently' do
+    ClimateControl.modify(
+      FEATURE_GEODATA: 'true',
+      FEATURE_BOOLEAN_PICKER: 'false'
+    ) do
+      assert Feature.enabled?(:geodata)
+      refute Feature.enabled?(:boolean_picker)
+    end
+  end
+end