Handle joins with preloads

[astjohn]

Further to #44, here is a pull request that attempts to fix queries that have joins with preloads. Implementation was provided by @TisButMe a while ago in this PR but it seems like it never made it into recent versions.

The caveat is that it was necessary to avoid any queries with group_by.

Please let me know what you think.

[drewolson]

Apologies for the delay in review. I've been thinking a lot about this PR and I'm honestly unsure if we should merge it. I much prefer the current subquery strategy as it counts the exact query being run (without the uncountable clauses). I also feel like it is a bit of an anti-pattern to rely on include to perform a join and modify the count of the result set.

Thoughts?

[astjohn]

No problem at all. I'm also swamped so apologies not necessary! Maybe it's just my naivety then and a matter of documentation.

How would we make the test pass that I added on line 355 of query_test.exs using a subquery? At the moment, it will fail without these changes.

[astjohn]

Hey @drewolson . Coming back to this... I just realized you closed it, but did you get the test I added to pass using a subquery. Would love to know how that works. Thanks.