Even though we currently only should have one group for a cluster, we
allow the flexibility to associate to other groups in the future.

This also matches the runner <=> groups association.

- Adds Cluster#first_group, aliased to Cluster#group. For the
conceivable future, a cluster will have at most one group.

- Prevent mixing of group and project clusters. If project type
clusters, it should only have projects assigned.  Similarly with groups.

- Default cluster_type to :project_type. As it's very small table we can
set default and null: false in one release.

This allows us (and others) to test drive Puma without it affecting all
users. Puma can be enabled by setting the environment variable
"EXPERIMENTAL_PUMA" to a non empty value.

This makes sure the user_status for the user of a label event is
preloaded. Otherwise it would not be rendered in the fake system note.

This functionality is needed for SSH push mirroring support, which is a
CE feature.

Before we would need to identify a user when pushing
through the GitLab UI. Since this is no longer the case
we can remove the identification by commit and instead,
use the identify_using_user

When Gitlab::GitPostReceive#changes_refs is empty
user would not get defined and nil would be passed
to PostReceive#after_project_changes_hooks which would
then throw an error.

Validating each commit on ChangeAccess times out if
it already took too long to complete.

Improves the TimedLogger specs to not make use of a stubbed
class anymore