Ported mdn-keystrokes.

* Keystrokes to command mapping landed in config.js.
* Blocked keystrokes landed in config.js as well.
* Toggle block command extracted to mdn-toggle-command plugin.
* Custom tab key functionality replaced by CKEditor's tab plugin and indentlist. It might not be 1:1 change, but the rest of custom tab handling seemed to be broken or unmaintainable.
Ported mdn-sampler plugin.
Copied tablesort plugin from CKEditor 3 to 4.
Aligned tablesort's lang file to CKE4.
Updated custom CKEditor build.