Apply TinyMCE security fix for CVE-2012-6112

Apply TinyMCE security fix for CVE-2012-6112 from
https://github.com/tinymce/tinymce_spellchecker_php/commit/22910187bfb9edae90c26e10100d8145b505b974.
Added the meta generator tag to the back end templates
Merge pull request #5288 from taca/tinymce-CVE-2012-6112/3.0.4
Correctly determin the last run of the command scheduler (see #5278)
Do not rewrite requests for .mp3, .mp4, .webm or .ogv files (see #5258, #5284)
The media content element now supports .ogg files (see #5282)
Removed the "spaceToUnderscore" option from all alias fields (see #5266)
Do not select more than required to get the new position of a record (queries are uncached!)
Fixed the "Call to a member function row() in News.php" bug (see #5265)
The jQuery accordion script did not work with minified markup (see #5245)
Updated SwiftMailer to version 4.3.0 (see #5263)
Updated SimplePie to version 1.3.1 (see #5207)
Added an option to load model relations uncached (see #5248, #5102)
Show the newsletter list even if there is no jumpTo page configured in the channel and show the enclosures in the newsletter reader (see #5233)
Added the `UserGroupModel` class to the autoload file
Added the theme store link
Allow users to edit comments of news archives and calendars even if they are not allowed to access the news or calendar module (see #5174)
Handle `@` blocks when importing style sheets (see #5250)
Handle `@` blocks when importing style sheets (see #5250)
Reverted b7c15ba185b42613b82f3fdc1bd5074a19d07573 and moved it to the develop branch (see #5174)