Partially addresses #47424.

Enum in Rails 5 does not map nil values - IOW nil value remains nil,
even if there is a key with nil value in the enum definition.

This commit overrides the underlying Enum methods so nil value is
still mapped. This solution is far from being ideal: it uses dynamic
definition of methods which introduces more magic/confusion
into the codebase.

It would be better to get rid of the nil value in enums.

methods

Includes Presentable module into CommitStatus. This fixes presenter
methods being called on those classes.

Closes #46177

This reverts commit 4f2cdb51.

This makes grouping on version number available. In general I'd like to
group based on MAJOR.MINOR version numbers for Gitaly.

Currently we still need to run EnsureStageService within a transaction,
because when it runs within in a transaction we are going to stick to
the primary database when using database load balancing. Extracting this
out of the transaction makes it possible to hit into problems with
replication lag in pipeline commit status API, which can cause a lot of
trouble.

- Fix spec

We still didn't migrate `ci_builds.stage_id`, so we can't use a
belongs_to association. We also have `ci_builds.stage` string attribute,
that we need to phase out in favor of `ci_stages.name`.

Older pipelines can contain jobs without a name, in which case
'nameless' is used right now, so we can properly still handle other
actions on these jobs.

Fixes gitlab-org/gitlab-ce#33403

These were untested by the cherry picked commit.