Style up community posts for bookmarks

[Former user]

Clicking on the "community post" button of a bookmark post should show a message informing the user that no community post exists yet, and that the user can create one (similar to publication community posts). If a community post exists, the post can be edited. this edit page already exists but needs to be styled.

[Daniel Zoller]

a larger screenshot of the community post would be nice

[Former user]

please use the import from the regular editBookmark page (no tags and groups as community posts don't have them)

[Sebastian Böttger]

Bootstrap style adaption done with commit 52d4b16.

[Former user]

implemented with 52d4b16