Implement module for setting "Poll-Interval" response header.
Return 429 in ETag caching middleware when polling is disabled.

See !10263

From Bullet failures in !10263

When deleting a user, the following sequence could happen:

1. Project `mygroup/myproject` is scheduled for deletion
2. The group `mygroup` is deleted
3. Sidekiq worker runs to delete `mygroup/myproject`, but the namespace and routes have
   been destroyed.

Closes #30334

infos on admin->monitoring->background

Looking at the SQL log, we see useless queries such as:

```
D, [2017-03-22T03:25:00.726710 #2629] DEBUG -- :    (235.9ms)  SELECT MAX("project_authorizations"."access_level") AS maximum_access_level, "project_authorizations"."user_id" AS project_authorizations_user_id FROM "project_authorizations" WHERE "project_authorizations"."project_id" = 13083 AND 1=0 GROUP BY "project_authorizations"."user_id"  [["project_id", 13083]]
```

fix lint issues

- Migration in Repository#is_ancestor?

Signed-off-by: NRémy Coutable <remy@rymai.me>

Updated some Ruby code based on comments

Prevent private group disclosure via parent_id

See merge request !2077

Use full path for moving directories when changing namespace path

See merge request !2078

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/26595