Assorted website updates

[jodygarnett]

• added social media to the community support page so we could link to fosstodon
• sponsors page (no longer in the wiki)
• add core contributors and sponsors to the home page
• fix a bunch of links
• the fork me on GitHub ribbon was a broken image (not sure why)
• removed nabble search
• irc to matrix
• generic foss4g ribbon so we do not need to update each year

Here is what the new supported by heading looks like:

Sponsorship page has the same text as the wiki.

[bradh]

LGTM.