Support sorting cluster by related model fields

[rjmackay]

• Support sorting a cluster by a related model field ie. vocalist__name
• Support sorting a cluster by a related model field ie. vocalist (where vocalist is a FK)
• Add tests

I'm not 100% sure how well this performs if those fields aren't loaded from the DB. I think potentially it'll hit the DB in an n+1 query.

Fixes #45

[rjmackay]

Ah looks like tests failed because I used f-strings which don't existing in Python 3.5. Happy to fix that but I'd love a review of the general approach first.

[rjmackay]

@gasman any update on getting this reviewed?

[ababic]

Hi @rjmackay. Since we need to be able to do something similar for filter strings, I thought it might be good to solve this in a more generic way. In #148, I've gone with an extract_field_value() function that can be reused here, as well as in the various value checking methods in queryset.py.

I haven't gotten round to writing tests yet, but what you've done here looks good. Do you mind if I copy that bit? Also, any thoughts you have on #148 would be appreciated.

[gasman]

Implemented in #148.