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
Update mybb_theme.xml
Update contact.lang.php
Update mybb_theme.xml
Update contact.lang.php
Fixes #330 Permaban Warning Levels don't work
Fixes #325 Thread Prefix Removed During Custom Mod Action
Update contact.php
Merge pull request #1016 from demonate/patch-4

#997 Work on Contact Page
Remove unnecessary "/"
Revert "#997 Work on Contact Page"
Merge pull request #1031 from mybb/revert-1016-patch-4

Revert "#997 Work on Contact Page"
Fixes #936 Wrong compatibility check for plugins
Merge pull request #1036 from PirataNervo/featurefix-936

Fixes #936 Wrong compatibility check for plugins
Fixes #369 Fixes #369 LIMIT clause in session updates

#369
Merge pull request #1038 from PirataNervo/featurefix-369

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

#338 1.8
Fixes #321 postsperpage negative value SQL error
Fixes #321 threadsperpage negative value SQL error
Merge pull request #1041 from PirataNervo/featurefix-321

Fixes #321 threadsperpage/postsperpage negative value SQL error