Enable sort by column inside associated record (BelongsTo)

[w0st]

Hello!

I've have a proposition for solving problems like this one: #1188

If your dashboard has a field which is a subclass of (or it is a) Administrate::Field::BelongsTo then these changes allows you sort by column inside associated record based on order option.

I would appreciate your comments

[nickcharlton]

This is looking good! Do you think you'd be able to add some tests for this feature, too?

[horacio]

@w0st Hello! I'm interested in helping completing this PR. Let me know if you'd like me to write the tests for this new functionality. Thanks!

[w0st]

Thanks for reminding me about this PR. I'll check the existing test suite in Administrate and notify you about the status. I remember that I spotted small problems with existing unit tests when I was creating this PR. I wanted to fix them before this.

[pablobm]

An alternative would be to implement #1506 (still in discussion; may not make it through).

[nickcharlton]

@w0st, where you able to look at this one again?

I've not looked into #1506 properly myself, but what do you think about that when you compare it to this approach?

[pablobm]

Superseeded by #2208