[ticket/11711] Inform user of unsupported characters while posting

PHPBB3-11711
[ticket/11711] Improve checks for unsupported characters and check subject

PHPBB3-11711
[ticket/11711] Reduce size of tests and also test topic titles

PHPBB3-11711
[ticket/11711] Improve coding and comments of character check

PHPBB3-11711
Merge pull request #2566 from marc1706/ticket/11711

[ticket/11711] Inform user of unsupported characters while posting

* marc1706/ticket/11711:
  [ticket/11711] Improve coding and comments of character check
  [ticket/11711] Reduce size of tests and also test topic titles
  [ticket/11711] Improve checks for unsupported characters and check subject
  [ticket/11711] Inform user of unsupported characters while posting