about page: move software section to bottom, add info about feedback
remove djangodash references from home view
deal with not responding nameservers (offline or unreachable)

if a nameserver does not respond, we flag it in the database
(available = False) and stop talking to it for 5 minutes.

after 5 minutes, we flag it available = True again and retry talking to it
(and if not successful, flag it as unavailable again).

this avoids a unresponsive UI as we do a lot of ns queries from the templates
to get v4 and v6 addresses of our hosts from master nameservers.

we log a warning when setting unavailable and a info when setting available.
rename Domain field available_for_everyone to public (so it is not confused with "available" field)

"public" field means that this zone can be used by every user (not just the owner) to create hosts there

"available" field means that the nameserver is operating and reachable