Add Gitter chat to the website

[InAnYan]

I wanted to go to JabRef's Gitter chat, but couldn't find it anywhere.

There is no Gitter link. I think it should be added in the footer in "Help & Contact".

I don't know how to link properly, but it's probably: https://matrix.to/#/#JabRef_jabref:gitter.im

[InAnYan]

This might be a good first issue, but I don't have permissions on this repo