Updates Code of Conduct source, page, and location

[kattni]

THIS PR REQUIRES beeware/.github#254 BE CONCLUDED BEFORE MERGING.

• Updates lektor_beeware_plugin to pull the Code of Conduct from the canonical location in beeware/.github/CODE_OF_CONDUCT.md. Uses the raw content URL to pull in the Markdown content, and converts it to HTML, to be injected into a Lektor Jinja template.
• Adds code-of-conduct.html template file to handle CoC content injection.
• Moves community/behavior/code-of-conduct to community/code-of-conduct.
• Adds link to Code of Conduct to site footer.
• Redirects community/behavior, community/behavior/code-of-conduct, community/behavior/making-a-coc-complaint, community/behavior/making-a-coc-report, and community/behavior/changes to appropriate locations (either the new primary Code of Conduct page, or the anchor to the associated section of the updated CoC).

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct