Bugfix: Only one `<main>` tag per HTML document

Only one `<main>` tag is allowed per HTML document. 
This change removes the ones in `web-container` and `web-runtime` and 
adds one to each extension (files-list, mediaviewer, markdowneditor, drawio) 
since they can't be loaded at the same time.

https://github.com/owncloud/web/issues/1652
https://github.com/owncloud/web/pull/4627
