Sort entries by -last_modified in monitor/changes changeset endpoint

[leplatrem]

Mimic behaviour of monitor/changes records endpoint

[leplatrem]

If we modify how changeset sorts things, there might be a few other impacts to think about. Like the remote-settings-devtools users. But that would be a clean solution if we make everything use changeset and it sorts things in an expected way.

You're right, clients would be instantiated by order of modified (collections with oldest last change would end up at the bottom). I think it's fine :) but let's look at this next year :)