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
Fixes #330 Permaban Warning Levels don't work
Fixes #325 Thread Prefix Removed During Custom Mod Action
Fix icon location in UTF-8 Conversion 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 #338 Spam notification for users with limited administrator permissions but no modera...

#338 1.8
Fix #1025 feature

https://github.com/mybb/mybb/issues/1025
Merge pull request #1043 from Destroy666x/fix1025

Fix #1025 feature
Merge pull request #884 from Nik101010/fix-836-feature

Fix: Length of guest usernames not checked [#836]
Merge pull request #1040 from PirataNervo/featurefix-338

Fixes #338 Spam notification for users with limited administrator perms
Fixes #1012 spinner image hardcoded

#1012
Merge pull request #1047 from PirataNervo/featurefix-1012

Fixes #1012 spinner image hardcoded
Fixes #999 theme editor not working JS

#999
Fixes #1051 Deleted custom profile fields still shown on profile
fixlastnameissue (reverted from commit 704aadbb3929a500af4769fcf76d0f1e3321c262)
Fixes #870 Not any last poster name in forumdisplay after splitting a/some post(s)