Metrics: Allow to search for relays per continent

During the last relay meetings it was pointed out that one goal is to increase the relay diversity around the world so I went to check for relays on other continents.

Currently the metrics interface only allows to see relays for each country but not for continents.
Also it would be nice to be able to query for more than one country at once. At the moment only the first is shown: https://metrics.torproject.org/rs.html#search/country:al%20country:dz

(if you prefer i can file a separate issue for the second one)

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