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 #990 - Quick Reply box gets shaded af...

#990 - Quick Reply box gets shaded after posting a reply
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 "/"
Fix icon location in UTF-8 Conversion page
Revert "#997 Work on Contact Page"
Merge pull request #1031 from mybb/revert-1016-patch-4

Revert "#997 Work on Contact Page"
Update contact.php
Update contact.lang.php

Added Subject and Message descs.
Update mybb_theme.xml

Modified 'contact' and 'post_captcha' templates.
Update contact.php
Fixes #936 Wrong compatibility check for plugins
Merge pull request #1036 from PirataNervo/featurefix-936

Fixes #936 Wrong compatibility check for plugins