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

Slightly changed and combined work of @thuvalpakshi
chore(composer): Rely on composer autoloader to load core classes
Merge pull request #7074 from ewinslow/composer-autoload

chore(composer): Rely on composer autoloader to load core classes
docs(all): fixes typos and improves readability
fix(views): respect icon_sizes config values when rendering icons

Remove hard-coded references to available icons sizes and use global icon_sizes config values and keys instead
Merge pull request #7088 from hypeJunction/icon-sizes

fix(views): respect icon_sizes config values when rendering icons
docs(cron): adds RST documentation about cron jobs

Converts wiki cron page to RST and references cron in Upgrading docs

Refs #6884
docs(all): improves formatting and comprehensibility of docs

Reworded some paragraphs to make more sense to new users. Applied formatting to some lists and paragraphs
Merge pull request #7087 from hypeJunction/docs

Fixes typos and adds cron RST docs
chore(composer): Stop checking in dependencies

BREAKING CHANGE:
If you are checking out Elgg directly from GitHub,
you will need to run `composer install` after `git checkout` to
get your installation to a working state.
Merge pull request #7083 from ewinslow/composer-ignore-vendor

chore(composer): Stop checking in dependencies
chore(composer): install sniffs via composer instead of PEAR
Merge pull request #7081 from ewinslow/composer-phpcs-1.x

chore(composer): install sniffs via composer
fix(embed): Manually load CSS/JS libs for embed when editing comments on the activity page.

Fixes #6422
Merge pull request #7107 from brettp/embed_activity_ajax

fix(embed): Manually load CSS/JS libs for embed when editing comments on...
fix(embed): Checking for lightbox and embed before loading JS libs when requested through AJAX.

Refs #7107.
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