Allow longer & shorter usernames, complying with the MXID length spec #1279

Job	Run time
Check frontend style	1m 3s
Run the frontend test suite	49s
Check schema	4m 40s
Run `cargo deny` checks	27s
Lint and test OPA policies	9s
Compile test artifacts	3m 33s
Check Rust style	12s
Check syn2mas	13s
Run Clippy	2m 16s
Run test suite with Rust stable (1)	2m 13s
Run test suite with Rust stable (2)	1m 56s
Tests done	2s
	17m 33s

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