Update upgrade file from db ver 1 to 2

Also allow the creation of debrepo even when not on gh-pages branch.
Update last_modified timestamp on db updates

Any update to the ip_net_plan table will now update the timestamp of the
prefix being updated.

Part of #367.
Improve wording and update db version number

The wording for the questions asked to the user have been reworded
somewhat.

Database structure number in nipapd.config is updated to the latest (2).

Part of #367.
Add new timestamp values to prefixes

The backend now serves two new values, the 'added' and 'last_modified'
columns of the database which are the respective timestamps for when a
prefix was first added and last modified. Both values are also
searchable via the prefix smart search function. Neither value is
writable as the backend updates them automatically.

xmlrpclib, both in the backend and in pynipap, is used with
use_datetime=True which means that the timestamps will automatically be
converted into datetime objects for easier use.

Pynipap does naturally not support saving the values as they are
read-only. The CLI will show the 'added' and 'last_modified' timestamps
when viewing a prefix. At this time, they are mostly meant for internal
usage or via the API, e.g. to bring up all prefixes that have been
changed the last hour to sync with another system.

Fixes #367.
Reverse previous accidental commit

This is reversal of some code that was mistakenly commited together with
a bunch of other code.
Add children column

The children column will be used to cache the number of direct
sub-prefixes (children) to a prefix. It is used by the web front-end to
determine if a expand/collapse icon should be shown or not.

Part of #421.