Proposal: add `civicrm_contact.image_file_id` column

Problem

Handling of contact images is nonstandard and difficult to work with.

Current Behavior

Uploading a contact image will:

1. Store the file in (by default) civicrm/custom.
2. Update the civicrm_contact.image_URL column with a link to an ajax callback for accessing the image.
3. Not create a record in the civicrm_file table.
4. Not create a record in the civicrm_entity_file table.

Deleting a contact image in the UI will:

1. Not delete the file.
2. Set civicrm_contact.image_URL to null.

Proposed New Behavior

Add a civicrm_contact.image_file_id column, FK to civicrm_file.id.

Uploading a contact image will:

1. Store the file as before.
2. Update the civicrm_contact.image_URL column as before.
3. Create a record in the civicrm_file table.
4. Store the file id in the civicrm_contact.image_file_id column
5. Undecided: should an entry also be created in civicrm_entity_file? See #2760

Deleting a contact image in the UI will:

1. Delete the file if an id is present in image_file_id column.
2. Set civicrm_contact.image_URL and civicrm_contact.image_file_id to null.

See PR https://github.com/civicrm/civicrm-core/pull/21141