This prepares the service and worker classes for post-processing of
group wikis, though we're not actually implementing any custom behaviour
for group wikis yet.

Wiki models:

- Move `HasWiki#after_wiki_activity` into `Wiki#after_wiki_activity`,
  and add `Wiki#after_post_receive` for container-specific background
  processing.

PostReceive worker:

- Move project-specific code into `ProjectWiki#after_post_receive`.
- Tweak check for project snippets with a missing project, to avoid
  catching group wikis.
- Remove redundant specs for ES indexing, which is already covered
  elsewhere.

Git::WikiPushService (also in EE):

- Change the service to take the wiki as its main argument.
- For group wikis, don't attempt to create events, index Elasticsearch,
  or run Geo services.