Add get_action_links to Widgy owners

Models with WidgyFields should define a get_action_links method. It
takes a root_node and returns a list of links. This is used for preview
links.

  - Removes the dependency in core on widgy_mezzanine for preview links
  - Removes the VersionCommitAdmin.get_commit_name and
    get_commit_preview_url methods.
  - Removes an attempt at making forms.WidgyField work for non-model
    forms. We'll revisit this when it comes up.
  - Adds preview and diff links for every owner of a VersionTracker.