elastic: UI indexing doesn't index the wiki

Spotted with @toupeira . Thanks for picking up on this!

The Elastic::IndexRecordService is meant to index all records relating to a project. It's used when indexing from the frontend, or anywhere that isn't sources from the rake itlab:elastic:index_wikis task.

Unfortunately, it completely forgets to index wikis!

We need to add this here before we can reliably self-serve elasticsearch on GitLab.com, and before we can think about removing the wiki indexing rake task. Anyone using the new indexing processes will find that their wikis just aren't searchable :/

I don't think this is needed for %12.0 but I do think we should tackle it with high priority @DouweM @phikai