Automatically enable multiple MR assignees feature flag

What does this MR do?

Schedules a background migration that checks whether multiple_merge_request_assignees feature flag needs to be enabled, otherwise it'll be rescheduled.

The rule for enabling it is that every PopulateMergeRequestAssigneesTable background job cannot be still running, or being retried or dead.

If there are any enqueued jobs in the state above, the "checker" (MergeRequestAssigneesMigrationProgressCheck) will be rescheduled for 3 hours ahead.

This MR doesn't depend on the feature code MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10161, given that the population migration MR was already merged (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26496). If the feature flag gets enabled automatically, that's fine, because the feature code is not there yet and we can disable it if we want. That's mainly needed for self-hosted instances.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer