Move start of the maintenance mode to the ajax call

Make sure the update page is shown in a browser. And not an ajax request
Connect watcher failure function to the failure signal
Also check for needed upgrade in the Sabre Maintenance connector
Fix inexisting trashbin's sub-folders
duplicate verification
Turn off theme before update
Add undefined verision variables
Merge branch 'master' into move-maint-start
Enable https authentication
Enable https authentication

Allow authentication using https:// as well as http://
Remove http:// from label and add placeholder
Update settings.php
Clear opcode caches after writing to the config file, fixes #3372
Update to jquery.multiselect 1.13
Forgot the css.
seperate mimetype guessing from filename
fix variable name
actually return result
Let's just use '/' as we do almost everywhere - this change fixes two failing unit tests
[Fixing Updater Unit Tests on Windows]
using $internalPath within call to self::correctFolder() because $path inside of it is not processed properly due to directory separator on Windows.
error logging has been added  in case the given 4path is not found within self::correctFolder