Handle invalid user IDs even better

[turt2live]

We previously fixed a bug where user ID handling wasn't perfect. This improves it further by checking more of the GMSL code paths, and making the check happen very early in the event check code. This allows filters to drop their custom handling. We also copy the behaviour across the rest of the user ID validation sites for safety.

Fixes #82

We're trying to validate this GMSL path specifically, which swallows errors: https://github.com/matrix-org/gomatrixserverlib/blob/20c9de33969e2df883eaa69c283d6a38efce9284/spec/senderid.go#L72

Tests aren't added for this because we can't mock the data without fighting that very code path.

Pull Request Checklist

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