Allow for filtering by multiple tags

[dergigi]

Right now we can only filter by one tag, as in:

• https://opensats.org/tags/core
• https://opensats.org/tags/nostr
• https://opensats.org/tags/grants
• https://opensats.org/tags/lts
• https://opensats.org/tags/funding

It would be great if we could filter by multiple tags, as in:

• https://opensats.org/tags/nostr,funding
• https://opensats.org/tags/core,grants
• https://opensats.org/tags/core,grants,lts

Implementing this might be a bit involved, as we probably would have to pre-generate all permutations n layers deep, since the stuff is static; but maybe I'm wrong and there's a much easier way to do this? Not sure...