Use a queue when deleting a large set of flaggings

Problem/Motivation

Split from #2489992: Do not delete flaggings on account cancelation

Deleting a large amount of flaggings can cause a project to run out of time / memory.

Steps to reproduce

Create a flag and add a high number of flaggings (e.g. 50k).

Attempt to delete the flag from /admin/structure/flags.

The operation will fail when PHP runs out of memory while attempting to delete all related flagging entities.

Proposed resolution

Use a queue when deleting a large set of flaggings

Remaining tasks

1. Write a merge request
2. Review
3. Commit

User interface changes

None

API changes

Large amount of flaggings are deleted in a queue

Data model changes

None