Delete flag's flaggings in a batch on entity delete

Problem/Motivation

When attempting to delete a flag with thousands of flaggings, the regular core entity delete behavior will run out of memory attempting to delete all the flaggings.

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

Delete a flag's flagging entities in a batch operation.

Remaining tasks

Review and improve initial approach. I have attempted to use as much as the extending entity delete functionality as possible, but it does lead to some oddities (e.g. the entity deleted message and log is added before the batch operation completes).

User interface changes

None.

API changes

None.

Data model changes

None.