Allow longer & shorter usernames, complying with the MXID length spec #1282

Job	Run time
Run `cargo deny` checks	31s
Compile test artifacts	3m 43s
Check schema	5m 14s
Check frontend style	46s
Run the frontend test suite	49s
Lint and test OPA policies	6s
Check Rust style	12s
Check syn2mas	13s
Run Clippy	2m 14s
Run test suite with Rust stable (1)	1m 59s
Run test suite with Rust stable (2)	1m 58s
Tests done	1s
	17m 46s

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