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

Fixes gitaly#629

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

The types `timestamp` and `datetime` are aliases:
https://github.com/rails/rails/blob/v4.2.10/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb#L362-L364

changed how the container class is added onto the text element
more specs

added back in documentation comment to flash file