Kubernetes deployments on new clusters will now have
a separate namespace per project environment, instead
of sharing a single namespace for the project.

Behaviour of existing clusters is unchanged.

All new functionality is controlled by the
:kubernetes_namespace_per_environment feature flag,
which is safe to enable/disable at any time.

API endpoints for requesting container repositories
and container repositories with their tag information
are enabled for users that want to specify the group
containing the repository rather than the specific project.

`allow_local_requests_for_hooks_and_services` was renamed to
`allow_local_requests_for_web_hooks_and_services`.

Add exclusive lease to mergeability check process

See merge request gitlab-org/gitlab-ce!31082

Concurrent calls to UserMergeToRef RPC updating a single ref
can lead to an opaque fail that is being rescued at Gitaly.

So this commit adds an exclusive lease to the mergeability
check process with the key as the current MR ID.

- DB migration of board milestone values
- issue finder & spec updates