feature(groups): Added featured groups as a tab along with groups, popular, discussions

Slightly changed and combined work of @thuvalpakshi
chore(context): Pull `elgg_*_context` functions' logic into a service
Merge pull request #6882 from Srokap/pr/6306

feature(groups): Added featured groups as a tab along with groups, popul...
Merge pull request #7101 from ewinslow/context-service

chore(context): Expose `elgg_*_context` functions as a service
chore(install): Move installer classes to classes dir

This allows the classes to be autoloaded just like all the others.

Fixes #7111