Previously, when the wiki page format was anything other than `markdown`
or `asciidoc` the formatted content would be returned though a Gitaly
call. Gitaly in turn would delegate formatting to the gitlab-gollum-lib
gem, which in turn would delegate that to various gems (like RDoc for
`rdoc`) and then apply some very liberal sanitization.

It was too liberal!

This change brings our wiki content formatting in line with how we
format other markdown at GitLab, so we have a SSOT for sanitization.

https://gitlab.com/gitlab-org/gitlab/issues/30540

Although `flat_map` is equivalent to `map` + `flatten(1)`
(note the level 1) we can apply this same refactoring to all cases.

Inside a wiki, when we show the sidebar or browse to the `pages`,
all page contents are retrieved from Gitaly and that is a waste
of resources, since no content from that pages are going to be
showed.

This MR introduces the method `ProjectWiki#list_pages`,
which uses new wiki_list_pages RPC call to retrieve
pages without content

Also in the `WikisController` we're using the method to show
pages in the sidebar and also on the `pages` page.

If a page has the same name as a directory
the WikiPage#group_by_directory method can have
a wrong behaviour

This commit fixes it

- Add controls for sorting by title and date
- Execute Gitaly call which now accepts sorting params for wikis

Adds `# frozen_string_literal: true` to spec/models ruby files

This reverts merge request !23229

This reverts merge request !23140

The disabling of Gitaly was basically stubbing `Feature.enabled?` call,
which was never triggered.

Currently, we assume a "last_version" always exists for a wiki page.
In production, this is not always true. So, guard uses of it with a
null check.

Cleanup code, and refactor tests that still use Rugged. After this, there should
be no Rugged code that access the instance's repositories on non-test
environments. There is still some rugged code for other tasks like the
repository import task, but since it doesn't access any repository storage path
it can stay.

Hooks were run for wikis, but given the internal API wasn't responding
failures happended, as seen on:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18693

Closes gitaly#958

Related to #39805

Part of gitlab-org/gitaly#639

Closes gitaly#673

The special characters of a wiki title are now escaped correctly.

Signed-off-by: NRémy Coutable <remy@rymai.me>

Remove superfluous lib: true, type: redis, service: true, models: true, services: true, no_db: true, api: true
Signed-off-by: NRémy Coutable <remy@rymai.me>