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 #338 Spam notification for users with limited administrator permissions but no modera

#338 1.6
Forgot something
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
Fix #1023 master

https://github.com/mybb/mybb/issues/1023
Merge pull request #1060 from Destroy666x/fix1023m

Fix #1023 master
Revert "Merge pull request #934 from PirataNervo/fix-870"

This reverts commit 75d65adb96c9c051743cd4cdad7136b230717162, reversing
changes made to f5e3b08f55f7273b6ec8ecf6dee293381850183b.
Merge pull request #1019 from PaulBender/fix-325

Fixes #325 Thread Prefix Removed During Custom Mod Action
Merge pull request #1017 from PaulBender/fix-330

Fixes #330 Permaban Warning Levels don't work
Fix #932 Odd code in usercp.php
Fixes #732 Modlog, $lang->post not defined

#732 master