feature(filestore): add elgg_get_file_simple_type() to core api

Consolidate simpletype parsing logic in a new elgg_get_file_simple_type() function added to core api
chore(docs): document simple_type,file hook
deprecate(filestore): deprecate file_get_simple_type() and file_get_general_file_type()

Deprecate file_get_simple_type(), found in mod/file/start.php,
and file_get_general_file_type(), found in engine/lib/filestore.php
in favour of a pluggable elgg_get_file_simple_type() added to core api
chore(cleanup): simplified docs and tightened code style
chore(refactoring): cleans up and simplifies logic in filestore lib
feature(messages): improved UI for messages listing

The checkbox for bulk actions was moved to the beginning of the row and
an excerpt of the message is shown in the listing

fixes: #4739
Merge pull request #7352 from mrclay/hypeJunction-simpletype

Adds elgg_get_file_simple_type() to core
Merge pull request #7343 from jeabakker/messages-checkbox-layout

feature(messages): improved UI for messages listing
feature(developers): inspect menu item now has children for faster
access

The way to inpect the different aspects of Elgg was quite a few clicks
away. Now all the options are children menu items of Inspect

fixes: #4831
feature(core): adds edit avatar to admin section of user hover menu

For easier access to the edit avator page, if a user has a inappropriate
avatar.

fixes: #5989
fix(river): rss layout supports mulitple installations in the same host

When there are more installations of Elgg on the same host the river
items would not be unique across the multiple installations.

fixes: #7019
feature(aalborg_theme): adds support for alert messages

Referencing  #7347
Merge pull request #7382 from jeabakker/river-rss-layout

fix(river): rss layout supports mulitple installations in the same host
Merge pull request #7389 from PerJensen/alert_messages

feature(aalborg_theme): adds support for alert messages
Merge pull request #7379 from jeabakker/developer-tools-inspect

feature(developers): inspect menu item now has children for faster access
fix(groups): adds wrapper to the message, This discussion is closed

Fixes  #7347
Merge pull request #7380 from jeabakker/edit-avatar-admin

feature(core): adds edit avatar to admin section of user hover menu
Merge pull request #7390 from PerJensen/message_wrapper

fix(groups): adds wrapper to the message, This discussion is closed
chore(di): Adopt PHP DI

Fixes #6387