Make writing relation count properties optional

[AegirLeet]

Accidentally fucked up #837 by deleting the branch or force-pushing (can't reopen), so here's a new PR.

I've rebased on top of current master, added a test for write_model_relation_count_properties and also added another test for write_model_magic_where while I was at it.

CC @mfn

[mfn]

Implementation looks fine, but the way I understood it, you added two tests:

1. one showing the default behavior, supposedly generating_count properties
2. one showing it can be disabled

Maybe I missed it, but it seems like only 2) works as intended as 1) (…\MagicWhere\…) doesn't show any _count 🤔

[AegirLeet]

The ModelsCommand/MagicWhere part isn't related to _count properties, I just noticed it was missing a test and decided to add it as well.

_count being added (default behavior) is already tested in some other tests, like in ModelsCommand/Relations. So ModelsCommand/RelationCountProperties only tests that they disappear when the new option is set to true.

[mfn]

Purrfect 🐱 :-)