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

Slightly changed and combined work of @thuvalpakshi
feature(discussion): Added some extension points at discussion sidebars.
These changes allow 3rd party plugins to extend discussion sidebar, and add
there features like a subscribe to discussion button, a list of most popular
posts or most active users...
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.
chore(tests): Add engine/classes as target for PHPUnit tests

This allows you to temporarily move the test file next to its source file
when you are actively developing and testing with it so that it's easier
to switch between two (which happens frequently in TDD).

The test should still be moved back to engine/tests/phpunit when committed.

Refs #7112
chore(install): Move installer classes to classes dir

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

Fixes #7111
Merge pull request #7110 from sembrestels/discussion-sidebar3

feature(discussion): Added some extension points at discussion sidebars.
Merge pull request #7122 from ewinslow/phpunit-src

chore(tests): Add engine/classes as target for PHPUnit tests
Merge pull request #7079 from ewinslow/composer-minify

chore(composer): Use composer to load mrclay/minify
chore(travis): Run coding standards, js, and docs builds in parallel
Merge pull request #7115 from ewinslow/installer-classes

chore(install): Move installer classes to classes dir
Merge pull request #7124 from ewinslow/travis

chore(travis): Run coding standards, js, and docs builds in parallel
chore(input): merge `get_input` and `set_input` into `Elgg_Http_Request`

Fixes #7091