[ticket/13215] Update Symfony components to v2.3.21.

PHPBB3-13215
Merge pull request #3062 from bantu/ticket/13215

[ticket/13215] Update Symfony components to v2.3.21.
Merge branch 'prep-release-3.1.0' into develop-ascraeus

* prep-release-3.1.0:
  [ticket/13215] Update Symfony components to v2.3.21.
[ticket/13207] Add default subscription options for newly registered users

PHPBB3-13207
[ticket/13207] Move default user notifications settings to user_add()

PHPBB3-13207
[ticket/13207] Add notification manager mock to user_add method in tests

PHPBB3-13207
Merge remote-tracking branch 'github-marc1706/ticket/13207' into prep-release-3.1.0

* github-marc1706/ticket/13207:
  [ticket/13207] Add notification manager mock to user_add method in tests
  [ticket/13207] Move default user notifications settings to user_add()
  [ticket/13207] Add default subscription options for newly registered users
[prep-release-3.1.0] Add 3.1.0 gold release migration
[prep-release-3.1.0] Add CHANGELOG for 3.1.0
[prep-release-3.1.0] Add missing CHANGELOG entries for RC6
[ticket/13221] Perform bot updates after user table changes

PHPBB3-13221
[ticket/13221] Add bot_update as a dependency to gold

PHPBB3-13221
Merge pull request #3073 from nickvergessen/ticket/13221

[ticket/13221] Perform bot updates after user table changes
[ticket/13223] Fix USERNAME email template variable in notification

admin_activate_user.php notification type is assigning usernames to email
template variables using user_loader->get_username() in 'no_profile' mode,
which in its turn calls get_username_string() function in 'no_profile' mode.
This causes HTML markup in emails for username colour.

PHPBB3-13223
Merge branch 'ticket/13223' into prep-release-3.1.0

* ticket/13223:
  [ticket/13223] Fix USERNAME email template variable in notification
[prep-release-3.1.0] Update CHANGELOG with last minute changes
[ticket/13225] Use passwords manager instead of phpbb_hash in migration

PHPBB3-13225
[ticket/13226] Fix the usage of $rank_img

PHPBB3-13226
[ticket/13226] Add functional test for group ranks

PHPBB3-13226
[ticket/13230] Add missing globals and missing request object for path helper

PHPBB3-13230