Make Onionoo base URL configurable.
Require existing relays to raise their bandwidth by 128 KB/s.

It shall be possible to participate with existing relays if their
bandwidth gets raised by at least 128 KB/s.

Implement this by looking at the advertised_bandwidth history in weights
documents.  If the maximum advertised bandwidth after the challenge start
is at least 128 KB/s higher than the advertised bandwidth before the
challenge, include that relay.  Also include all new relays, all relays
for which there is no such information available, and all bridges.