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...
chore(filesystem): Add basic filesystem abstraction backed by Flysystem

Refs #6803
Merge pull request #7101 from ewinslow/context-service

chore(context): Expose `elgg_*_context` functions as a service
Merge pull request #7113 from ewinslow/filesystem

chore(filesystem): Add basic filesystem abstraction backed by Flysystem
chore(composer): Use composer to load mrclay/minify

BREAKING CHANGE:
The CSSMin class included via minify was renamed to CSSmin.
If you were referring to it with capital M, you'll have to
change that to lower-case m.

However, note that Elgg's dependencies are not considered
public API, so this notice is only a courtesy. Please explicitly
declare your dependencies on third party vendors, even ones that
you know Elgg already includes. We may remove or update them at any time.