Group long names - help give full citizenship to groups for mentions, messaging, group pages

[tgxworld (Alan Tan)]

I just made group mention similar to user mention.

https://github.com/discourse/discourse/commit/43671b1fdac10817b89b4f2c8c96cf29c6dbf5d1

Also improved the autocomplete by adding the user’s name / group’s fullname to the title attribute

Thanks @tobiaseigen