I missed adding the rb7 intersphinx link, so links to the RB docs from

the release notes weren't building. We didn't notice because prior to

the 7.0 release, we didn't have any RB7 docs on the website anyway.
This also adds a warning about Django's dropped DB versions like we had

in the Djblets release notes.

Built the release notes and checked for errors in links to public docs.