Sort arrays used for comparison

Previously, you'd have to make sure your arrays of tags to search for were in
the same order as when they were saved.

Now they can be in any order and the comparison will still return them.
Ensure tests capture tag arrays in different order
Handle nil tagsets

If the tags don't exist in the record being compared we shouldn't throw an
error, we should just assume it doesn't match.

Side benefit: fixes CI.
Merge pull request #50 from alphagov/sort-subscriber-list-comparisons

Make where_tags_equal insensitive to array ordering