Implementation of sharing mechanisms for corpora and users.

• Option to mark a corpus as public visible (only for other logged in users)
• Page for exploring public corpora
• Rework corpus pages, so that the overview page gives more/better information about the corpus
• Option to follow a public corpus by an invitation link with expiration (this should be possible even if the corpus is not public)
• Option to follow a public corpus via button on public corpus list and corpus page
• Follower management
  • Permission management for corpus followers (ro, rw, ...)
  • Let corpus owners delete followships