Sorting preference functionality has been extracted
from `IssuableCollections` to a new `SortingPreference`
concern in order to reuse this functionality in projects
(and groups in the future).

In order to save user preferences regarding
user emails allowed to be invited to group

Add foreign_key and down method

Change adding foreign key

Add partial call to view

Add changelog entry

Fix schema

To index notes, we exclude system notes and use `find_in_batches` to
load them in batches for submission to the ES bulk import API.
These queries often result in DB timeouts because the usage of
`ORDER BY id` results in the `notes_pkey` index being used.

This adds an optimized partial index, and removes the unused index
`index_notes_on_noteable_type` which is already covered for our
usage by the existing `index_notes_on_noteable_id_and_noteable_type`.

Newer versions of PostgreSQL (at least 11) are smarter about this and
use `index_notes_on_project_id_and_noteable_type` instead, so we might
be able to remove the partial index again in the future.

This change lays the foundation for customizable cycle analytics stages.
The main reason for the change is to extract the event definitions to
separate objects (start_event, end_event) so that it could be easily
customized later on.

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

Backport of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14943

See merge request gitlab-org/gitlab-ce!31406

This makes `epic_issues` consistent with other tables
using our `RelativePositioning` class