- Adds UI to configure in group and project settings
- Removes notification configuration for users when
disabled at group or project level

These are not required because MySQL is not
supported anymore

Remove old ignore_column in AR models

See merge request gitlab-org/gitlab-ce!30725

Next models are affected:
* Project
* Namespace
* Issue
* Merge request
* CI Trigger
* CI Pipeline schedule

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

This gets overridden in `EE::Namespace` and allows us to do things like
always treat Epics as "disabled" in Core using the same checks we'd use
elsewhere.

Since we use `Namespace#path` to build routes and URLs we can lessen
the restrictions on `Namespace#name` so it can accomodate a user's name.

- Add two new ActiveRecord models:
  - RootNamespaceStoragestatistics will persist root namespace statistics
  - NamespaceAggregationSchedule will save information when a new update
to the namespace statistics needs to be scheduled
- Inject into UpdateProjectStatistics concern a new callback that will
call an async job to insert a new row onto NamespaceAggregationSchedule
table
- When a new row is inserted a new job is scheduled. This job will
update call an specific service to update the statistics and after that
it will delete thee aggregated scheduled row
- The RefresherServices makes heavy use of arel to build composable
queries to update Namespace::RootStorageStatistics attributes.
- Add an extra worker to traverse pending rows on
NAmespace::AggregationSchedule table and schedule a worker for each one
of this rows.
- Add an extra worker to traverse pending rows on
NAmespace::AggregationSchedule table and schedule a worker for each one
of this rows

The root_ancestor method is heavily used in EE for the group_saml
feature. Having this memoization implemented in CE would eliminate
the need of overriding the root_ancestor method in EE.

- Namespace::Storagestatistics will persist root namespace statistics
- Namespace::AggregationSchedule will save information when a new update
to the namespace statistics needs to be scheduled

Both tables use 'namespace_id' as primary key