Enhancement: Text Editor

Replace MarkdownEditor with Text Editor and add the following improvements:
* Disable preview for non md files (configurable)
* Data integrity checks (prevent exiting with unsaved changes)
* Better error information to the user
* Keyboard shortcut to save
* Read-only mode
* Fix to open on non-authenticated public links
* Open other extensions (js, json, xml, py, php, yaml)
* UI polish

https://github.com/owncloud/web/pull/6667
https://github.com/owncloud/web/pull/6754
https://github.com/owncloud/web/pull/6763
