Strict relay selections

[staab]

Right now, relay selections are a mess of heuristics and assumptions.

• Make relay quality less of a factor. Lots of times we use certain relays just because we already have a connection, even if they're completely irrelevant. Don't use relays that can't be connected to of course, but also don't be afraid to open more connections.
• Who even knows how fallbacks get selected. Use user read relays + static fallbacks.
• Show errors instead of guessing relays for DMs
• Respect user relay selections even if they're not normal URLs, but don't connect to non-public urls in other people's lists
• Follow pubkey selections, pubkey hints, and relay hints when loading context.

Blocked by #444 and #446, since those reduce the scope of the work significantly.