Some minor changes with regard to CKeditor 4.1 / #263

- Added some config options for codemirror.
- Removed "About" (sorry @electrolinux. CKEditor is credited on the
about page, the [?]-button would just confuse noob-users)
- Removed an unneeded file. (it was duplicated, and we kept the one
that's used)
Updated Codemirror to version 3.11.

The core files are now used from the ckeditor plugin, to prevent
duplicate files. Also added 'php' to editmode in Codemirror. Related to
#263.