[#6539] use _id (indexed) instead of full_slug as key when approving posts

In 93768a36 test_post_count was already rendered irrelevant, so removing
altogether.  Fixed up test_threads_with_zero_posts as best we can for now
[#6619] backwards compatibility for old tickets
[#6619] make h.absurl() have proper value in tasks

* add hostname to taskd fake request
* add hostname for tests
* request.host_url is better, it doesn't include port numbers
* add tests
[#6619] update all the tests using :80