- When renaming a column concurrently, drop any existing trigger before
attempting to create a new one.

When running migration specs multiple times (as it happens during
local development), the down method of previous migrations are called.
If any of the called methods contains a call to
rename_column_concurrently, a trigger will be created and not removed.
So, the next time a migration spec is run, if the same down method is
executed again, it will cause an error when attempting to create the
trigger (since it already exists). Dropping the trigger if it already
exists will prevent this problem.

Add to the service and migration both.

- Introducting StageEvents to define the available events
- Define the event pairing rules, since some events are not compatible
- Express default Cycle Analytics stages with the event structure

Add specs for new parameter and updated documentation as well.

Add frozen_string_literal to lib part 2

See merge request gitlab-org/gitlab-ce!32094

Root namespaces have storage statistics.
This commit allows namespace owners to get those stats via GraphQL
queries like the following one

{
  namespace(fullPath: "a_namespace_path") {
    rootStorageStatistics {
      storageSize
      repositorySize
      lfsObjectsSize
      buildArtifactsSize
      packagesSize
      wikiSize
    }
  }
}