We replace the usage of WikiPage::CreateService with the wiki_page factory in the specs.

We were looking these up on each request to an issue page, because the form is
pre-filled, as is the template dropdown. That was unnecessary: we could just
treat these as 'special' repository files (like the rendered README) and cache
them in Redis until they change on a push.

Fixes #39046

The basename appears to have been a holdover from the past - it doesn't look
necessary now. Some of the regexes were unanchored on one side, so explicitly
ensure that they only match the root.

Apart from that, this means that pushing to foo/README.md will no longer
invalidate the main README cache for a project.

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/38871

Collect usage pings `Gcp::Cluster.enabled` and `Gcp::Cluster.disabled`, instead of `Gcp::Cluster.count`

Fixes gitaly#629

This also refactor the email_helper support spec to watch for multiple
emails being sent.