Closes #12988: Introduce custom field choice sets

[jeremystretch]

Fixes: #12988

• Introduce the new CustomFieldChoiceSet model
• Automatically migrate all CF choices to the new model
• Add a choice_set ForeignKey to CustomField and remove the choices array field
• Introduce ArrayWidget to improve UX while editing choices

[DanSheps]

Line separated appears to have issues when used on Windows clients. Quick and dirty fix is to replace CRLF with LF only.

[jeremystretch]

Thanks @DanSheps. I'm going to hold off on the documentation updates as I'll need to rewrite the descriptions for extra_fields for #12194 anyway.

[arthanson]

Also noticed when you add a custom field "Choice set" is only valid if you have selection type - we should add a PR to change the form to use HTMX so "Choice Set" is only displayed if you have a selection type.

[DanSheps]

Thanks @DanSheps. I'm going to hold off on the documentation updates as I'll need to rewrite the descriptions for extra_fields for #12194 anyway.

Yeah, no problem.

The splitlines() is way better then my solution anyways, and I always forget about it.