Rely on viewer app to open single shared .md file

[max-nextcloud]

With https://github.com/nextcloud/server/pull/33347 and https://github.com/nextcloud/viewer/pull/1242 the viewer will be used to open single shared files. Text still has its own handling in src/public.js. Using the viewer to open the file just like we do in other places seems like the more generic approach.

Using the viewer as is does not work right now mainly due to css style issues. The text editor will have a hight of 0 and therefore not be visible. There's also other side effects of the surrounding page such as text-align: center that need to be accounted for.