Refactor PostReceive hooks for group wikis [RUN AS-IF-FOSS]

What does this MR do?

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 (that will follow in our MVC 2 epic).

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.

Ref: #207870 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team