Switch to the Contextly Kit. Re-factoring and fixes.

* Move most of the post editing features to a separate file.
* Merge TinyMCE plug-ins into a single one with all features.
* Deactivate all editor controls while data is still loading.
* Remove sidebar token from the TinyMCE on sidebar removal from the editor.
* Use overlay component on the admin side instead of the Thickbox.
* Hide Contextly button on the publish meta-box if widgets are disabled on the post.
* Render snippet editor button for the Contextly box on the server side.
* Add methods to return 404 and 500 HTTP status.
Add option to turn on/off Kit resources loading from the CDN.
Merge pull request #15 from rsingel/trt-kit

Kit integration