allow a channel owner to define a default mimetype on webpages (otherwise default to "choose") - also fix the selector w/r/t php code. There is no ui for setting either at the moment, but for instance with wiki pages we probably don't want to have multiple choice of the mimetype. Keep it simple.
only allow richtext editor on pages if bbcode is the default mimetype, otherwise use plaintext editor. This will eventually need work to support a visual editor for html or other types.
Merge https://github.com/friendica/red into zpull