rename some model attributes, add migration

Domain.domain to Domain.name
Host.subdomain to Host.name
BlacklistedDomain.domain to BlacklistedDomain.name_re
rename BlacklistedDomain to BlacklistedHost, add migration
rename domain_blacklist_validator to host_blacklist_validator
fix some places that relied on TEST_HOST being str

this prepares next commit
use FQDN namedtuple for test hostnames
add related hosts functionality, to update host records based on main host network address + interface id

this is esp. useful for v6 hosts, where your provider might give you a changing prefix (we assume /64), but your interface ids stay the same.

but same mechanism also principally works for v4 (we assume /29), except that you usually do not get a v4 network where the network address is changing.
and when it comes to dynamic addresses, many people only get 1 ipv4 address anyway. the related host could then be used for aliaseses with ifid == 0 for all aliases.
add user interface for related hosts
add pre_delete handler for RelatedHost to remove related hosts from DNS
add comments for related hosts
make netmasks / prefix lengths configurable in host record
rather use PositiveSmallIntegerField for netmasks
show related hosts' ipv4/v6 addr from nameserver
update docs about related hosts feature
fix netmask and pep8 in test setup