Change link to "search for a router" from metrics' relay-search.html to Atlas
The reason is that I'm planning to either move or, more likely, shut down the relay-search function, because it's one of the few things that require us to keep a full database of relays.
The main difference between the two searches is that relay-search.html will find relays that haven't been running for more than a week, which Atlas won't find. But that shouldn't matter much for Weather, because people typically want to register relays that are currently running.
This is only an intermediate solution. The better long-term solution is that Weather implements its own relay search function, maybe using Onionoo. But let's do the simplest thing for now and just remove the dependency on metrics' relay-search.