Add MIT and OGM GIS sources

Why these changes are being introduced:

Two new sources are getting added to TIMDEX: MIT and OpenGeoMetadata (OGM) GIS data.

How this addresses that need:
* Adding the new data source names to VALID_SOURCES allows them to be indexed to OpenSearch

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/GDT-92

Author: ghukill

tim/config.py

@@ -15,6 +15,8 @@
     "alma",
     "aspace",
     "dspace",
+    "gismit",
+    "gisogm",
     "libguides",
     "jpal",
     "researchdatabases",