* Hopefully fixes spec failures in which the table doesn’t exist
* Decouples the background migration from the post-deploy migration, e.g. we could easily run it again even though the table is dropped when finished.

Since `calculate_checksum` depends on `Uploader` classes which are not defined in this background migration and may change at any time.

This will prevent our other migration for adding old files to the uploads table from breaking.

I believe the field only needs to fit 519 at the moment but I’m rounding up to be a little safer.

See the migration spec for more detail on the magic number 519.

So the path on source installs cannot be too long for our column.

And fix the column length test since Route.path is limited to 255 chars, it doesn’t matter how many nested groups there are.

Because it is a temporary table meant only to facilitate a migration of data. It is referenced only by the post-deploy migration and 2 related background migrations.

It should be dropped when the data migration is finished.

Since duplicate inserts are now ignored.

by copy-pasting in the whole `Upload` class.

Also, fix `Namespace` `model_type` (it should not be `Group`).

To process the unhashed_upload_files table.

To create the table, and schedule the background migration that begins the work.

Documentation bug fixes: Added procedure to disable built-in Issues.

See merge request gitlab-org/gitlab-ce!15581

Merge branch '40746-broken-master-due-to-failure-in-spec-services-clusters-applications-schedule_installation_service_spec-rb' into 'master'

Ensure `Namespace`'s is namespaced in `Gitlab::Kubernetes::Helm#initialize` and fix a transient failing spec due to that

Closes #40746

See merge request gitlab-org/gitlab-ce!15684