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(php): convert class underscores to namespaces