Enhancement: Integrate ToastUI editor in the text editor app

We've integrated the ToastUI editor in our text editor app.
This makes writing markdown much easier, since the users will have access to a markdown compatible toolbar.
Code syntax highlighting is also supported.

https://github.com/owncloud/web/pull/10390
https://github.com/owncloud/web/pull/10467
https://github.com/owncloud/web/pull/10465
https://github.com/owncloud/web/issues/9495
https://github.com/owncloud/web/issues/10385
