This breaks all kinds of things for now, but will open up using the
`accessible_by` on scopes.

Rails 5.1 has been EOL for a long while, and the code hoops we'd have to
jump through to keep supporting it are really quite large.

This passes all the conditions we have into ActiveRecord's `where`
method and joins them using `or` and leaves the SQL generation to
ActiveRecord/Arel.

The conditions array is really an artefact of the old implementation and
should IMO not be tested. Let's instead test where the rubber hits the
road: Which database records are returned.

Now that we're using ActiveRecord to generate and merge conditions, we
do not need a lot of code any longer.

This extracts a lot of stuff into methods. I sure am hopeful this makes
the code more readable!

IDs have primary key constraints, and we cannot know when our test runs
respective to other tests.

Ok, so Postgres can't order by a column that's not on the select list.
Let's put the column on the select list if that's what we want to order
by!