Remove dependency on IgnorableColumn concern

Closes #66746

See merge request gitlab-org/gitlab-ce!32427

This is to accomodate prepended modules.

Filter out old system notes for epics in notes api endpoint response

See merge request gitlab/gitlabhq!3224

Limiting the size of issuable description and comments to 1_000_000,
which is close to ~1MB of ASCII characters, which represents 99.9% of
all descriptions and comments we have in DB at the moment. This should
help prevent DoS attacks when comments contain refference strings.

Also this change updates regexp matching the namespaces paths by
limiting the namespaces paths to Namespace::NUMBER_OF_ANCESTORS_ALLOWED,
as we allow 20 levels deep groups.

see https://gitlab.com/gitlab-org/gitlab-ce/issues/61974#note_191274234

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31117 enabled the
HashInefficientHash Rubocop rule that was fooled by the special
implementation of `SpecialRole`. We fix this by introducing a
`value?` method and adding unit tests.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65383

When used with a Hash, `.keys.include?` is bad because:

1. It performs a O(n) search instead of the efficient `.has_key?`
2. It clones all keys into separate array.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64975

A new `discussion_id` argument on the `createNote` mutation allows
people to create a note within that discussion.

The ability to lazy-load Discussions has been added, so
GraphQL.object_from_id can treat Discussions the same as AR objects and
batch load them.

https://gitlab.com/gitlab-org/gitlab-ce/issues/62826
https://gitlab.com/gitlab-org/gitlab-ee/issues/9489

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>