templater: add Email template type, deprecate `Signature.username()` #17087

Job	Run time
Build jj-lib without Git support	45s
build (ubuntu-latest)	5m 10s
cargo-deny (advisories)	25s
cargo-deny (bans licenses sources)	27s
build (macos-13)	5m 59s
build (macos-14)	3m 51s
Check protos	23s
Clippy check	2m 38s
build (windows-latest)	11m 7s
build (ubuntu-latest, --all-features)	6m 6s
Check that MkDocs can build the docs	8s
Check that MkDocs can build the docs with latest Python and uv	11s
Check formatting	17s
	37m 27s

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