Fix: Length of guest usernames not checked, refs #836

* inc/datahandlers/post.php
  * added verification of username for guests input using user datahandler
* newreply.php / newthread.php
  * added verification of username for preview
* inc/languages/english/datahandler_post.php
  * added language strings for possible error messages in post author validation
Fixes #330 Permaban Warning Levels don't work
Fixes #325 Thread Prefix Removed During Custom Mod Action
Fixes #936 Wrong compatibility check for plugins
Merge pull request #1035 from PirataNervo/fix-936

Fixes #936 Wrong compatibility check for plugins
Fixes #369 LIMIT clause in session updates
Merge pull request #1037 from PirataNervo/fix-369

Fixes #369 LIMIT clause in session updates
Fixes #338 Spam notification for users with limited administrator permissions but no modera

#338 1.6
Forgot something
Fixes #321 threadsperpage negative value SQL error

#321
Fixes #321 postsperpage negative value SQL error

#321
Merge pull request #1042 from PirataNervo/fix-321

Fixes #321 threadsperpage/postsperpage negative value SQL error
Fix #1025 master

https://github.com/mybb/mybb/issues/1025
Merge pull request #1044 from Destroy666x/fix1025m

Fix #1025 master
Merge pull request #885 from Nik101010/fix-836

Fix: Length of guest usernames not checked [#836]
Fixes #913 double escaping in functions_search.php

#913
Merge pull request #1045 from PirataNervo/fix-913

Fixes #913 double escaping in functions_search.php
Parse error fix #913

#913
Merge pull request #1046 from PirataNervo/fix-913

Parse error fix #913
Merge pull request #1039 from PirataNervo/fix-338

Fixes #338 Spam notification for users with limited administrator perms