[ticket/12648] Improve UCP/MCP/Posting tabs

PHPBB3-12648
[ticket/12648] IE8 & IE9 color fix

PHPBB3-12648
[ticket/12648] More style fixes and also cleaned up CP left navigation

PHPBB3-12648
[ticket/12648] Fix and modernize ACP header tabs

PHPBB3-12648
[ticket/12648] Added ARIA functionality

PHPBB3-12648
[ticket/12648] Fix poll tab display error

PHPBB3-12648
[ticket/12648] Fix poll tab and revert unrelated color change

PHPBB3-12648
[ticket/12648] Slightly clear ACP active tab border

PHPBB3-12648
[ticket/12648] RTL fix & ACP border fix

PHPBB3-12648
Merge remote-tracking branch 'PayBas/ticket/12648' into develop-ascraeus

* PayBas/ticket/12648:
  [ticket/12648] RTL fix & ACP border fix
  [ticket/12648] Slightly clear ACP active tab border
  [ticket/12648] Fix poll tab and revert unrelated color change
  [ticket/12648] Fix poll tab display error
  [ticket/12648] Added ARIA functionality
  [ticket/12648] Fix and modernize ACP header tabs
  [ticket/12648] More style fixes and also cleaned up CP left navigation
  [ticket/12648] IE8 & IE9 color fix
  [ticket/12648] Improve UCP/MCP/Posting tabs