[ticket/12982] Refactoring: made most JS nice.

PHPBB3-12982
[ticket/12982] Refactoring: made JS in adm nice

PHPBB3-12982
[ticket/12982] Refactoring: Cleaned up phpbb.search

PHPBB3-12982
[ticket/12982] Refactoring: Cleaned up forum_fn.js

PHPBB3-12982
[ticket/12982] Refactoring: Cleaned up crazy insert users stuff

PHPBB3-12982
[ticket/13005] Add event to display_forums() to modify category template data

While there're events to modify/add FORUM_POST/FORUM_LINK template data,
such an event is missing for FORUM_CAT forum type.

PHPBB3-13005
[ticket/13006] Add variables to the 'core.modify_quickmod_actions' event

Add $quickmod and $action variables
for the 'core.modify_quickmod_actions' event trigerring
to avoid of using global operator in corresponding extensions.

PHPBB3-13006
[ticket/13034] Fix the route generated for the frontend not in the phpbb root

PHPBB3-13034
[ticket/13034] Fix unit tests

PHPBB3-13034
[ticket/12976] Correctly use plurals for attachment count in UCP navigation

PHPBB3-12976
[ticket/12982] Refactoring: Killed undescriptive variables

PHPBB3-12982
[ticket/12963] Fix the list of migrations in database_update.php

Without this patch the finder grab all the class available in the ext
folder and not only the migrations. This change is backported for the one
done before to the cli tool db:migrate. (see the commit
3420f8f3201ac337434f73ee00bda6df7b378212)

PHPBB3-12963
[ticket/13042] Remove unused variables

PHPBB3-13042
[ticket/12734] Remove unnecessary error supressing

PHPBB3-12734
[ticket/12963] Don't use static var in \extension\base\get_migration_file_list

The static var was global to all instance of \phpbb\base and so if two
different instances (for two different extensions) were created by the
same script they shared the same migrations list.

PHPBB3-12963
[ticket/12963] Load extensions migrations from /migration

PHPBB3-12963
[ticket/12963] Add unit tests

PHPBB3-12963
Merge pull request #2936 from nickvergessen/ticket/12976

[ticket/12976] Correctly use plurals for attachment count in UCP navigation

* nickvergessen/ticket/12976:
  [ticket/12976] Correctly use plurals for attachment count in UCP navigation
Merge pull request #2940 from Senky/ticket/13042

[ticket/13042] Unused var in login_box()

* Senky/ticket/13042:
  [ticket/13042] Remove unused variables
Merge pull request #2903 from rxu/ticket/13006

[ticket/13006] Add variables to the 'core.modify_quickmod_actions' event

* rxu/ticket/13006:
  [ticket/13006] Add variables to the 'core.modify_quickmod_actions' event