[ticket/12788] Update symfony components

PHPBB3-12788
[ticket/12788] Force 2.3.* on symfony dependencies

PHPBB3-12788
[ticket/12816] Fix comment about logs in user_ban function

PHPBB3-12816
Merge pull request #2679 from dhruvgoel92/ticket/12788

[ticket/12788] Update symfony components

* dhruvgoel92/ticket/12788:
  [ticket/12788] Force 2.3.* on symfony dependencies
  [ticket/12788] Update symfony components
Merge pull request #2697 from n-aleha/ticket/12816

[ticket/12816] Fix comment about logs in user_ban function

* n-aleha/ticket/12816:
  [ticket/12816] Fix comment about logs in user_ban function
[ticket/12821] Convert .headerbar to css gradient

PHPBB3-12821
[ticket/12821] Convert .forabg to css gradient

PHPBB3-12821
[ticket/12821] Convert .forumbg to css gradient

PHPBB3-12821
[ticket/12821] Convert .forums to css gradient

PHPBB3-12821
[ticket/12821] Convert .button1-3 to css gradient

PHPBB3-12821
[ticket/12821] Fix .button gradient for consistency

PHPBB3-12821
[ticket/12821] Fix .activetab gradient for consistency

PHPBB3-12821
[ticket/12821] Fix #navigation gradient for consistency

PHPBB3-12821
[ticket/12821] Fix dropdown gradient for consistency

PHPBB3-12821
[ticket/12821] Do not repeat bg on .button1-3

PHPBB3-12821
[ticket/12821] Remove obsolete images

PHPBB3-12821
[ticket/12821] Fix header in IE

IE 9 Requires an additional wrap and dual border-radius with an overflow to combine gradients with border-radius

PHPBB3-12821
[ticket/12821] Fix all forums & topics section gradients in IE

IE 9 Requires an additional wrap and dual border-radius with an overflow to combine gradients with border-radius

PHPBB3-12821