feature(file): display file upload limit on file upload form

Refs #6355
feature(groups): Added featured groups as a tab along with groups, popular, discussions

Slightly changed and combined work of @thuvalpakshi
fix(embed): embed jquery target is now searched for instead of assuming last class
Merge pull request #6796 from beck24/embed_js

fix(embed): embed jquery target is now searched for instead of assuming last class
fix(notification): extract notification summary from $params

Fixes #6885
fix(replies): Show reply's link on river
fix(a11y): Label form fields in installer correctly

Refs #6743
fix(a11y): Use HTML5 form features on install forms

Email/Url fields are announced as such by screen readers
and prevent submission if not correctly formatted.

Required fields prevent submission of form if not filled in.
fix(a11y): Add semantic structure to installer page layout

aria landmark roles allow advanced screen reader users to
jump around the page quickly.

Aria roles are used in addition to semantic elements because
browsers do not yet consistently expose the semantic elements
as landmarks to assistive technology.

Ideally we would actually just shim this behavior with JS
so that we don't have to worry about consistently pairing
the elements with their equivalent roles.
Merge pull request #6924 from RiverVanRain/1.9

fix(replies): Show reply's link on river
Merge pull request #6931 from ewinslow/install-a11y

fix(install): Better accessibility for installation flow
perf(session): speed up elgg_is_admin_user()

This function currently makes one query and generally hits the query cache
several more times. This generally eliminates the query and avoids the overhead
of the DB service. If a query must be done we use a minimal result set.
Merge pull request #6947 from mrclay/is_admin_user

perf(session): speed up elgg_is_admin_user()
docs(i18n): internationalized the documentation

• Updated the Makefile to generate the templates in the locale folder.
• Added necessary fields to conf.py.
• Updated the Transifex configuration file to cover the documentation.
• Added *.mo and .po to .gitignore.
• Updated the documentation about writing documentation and releases.

Fixes #5899
fix(install): Make installer usable on smartphones
chore(install): Point docs links to learn.elgg.org instead of docs.elgg.org

Refs #6784
chore(i18n): update translations
chore(release): v1.9.0-rc.3
Merge pull request #6973 from Srokap/release-1.9.0-rc.3

Release 1.9.0 rc.3
fix(file): download adds header Content-Length