Check that entered hosts are part of the current site

* Add a raw_hosts method to bulk_adder to pull out an array of unique
hostnames used
* Check that each of these is a valid site hostname, otherwise return
an error
* Ignore paths

Caveat: This may be overly strict, eg with and without www. at the
start of the hostname.
Handle URI parse errors

* When URI parsing fails on what looks like a full URI then
site_has_hosts? should return false
* In tests distinguish between invalid and incorrect
Merge pull request #161 from alphagov/check-hostname-on-add

Check that entered hosts are part of the current site