Name branches after the account, and labels after the display

Name branches after the account, and labels after the display

A contributor's folder was `refs/heads/contrib/<account-id>/`, which told a
reader nothing without decoding a ULID. It is now the account name,

    refs/heads/contrib/<name>/<branch>

and the name is therefore fixed at creation: renaming an account would strand
every ref under its old folder, so `vole admin user rename` is gone and the
users row itself — kept after retirement — is what stops a name being reissued.

What people see is now a separate, mutable display name, validated by the same
rules wherever it is set (CLI, remote admin, registration, `/account`) and shown
on pages, approvals, listings and refusals. Nothing is authorized by it and
nothing on disk is keyed by it: metadata authors stay account names because the
push boundary resolves them for ownership, and approvals stay account ids.

`user_names`, `Snapshot.Reserved` and `Account.Former` are dropped: with a fixed
name there are no former names to remember, and the UNIQUE column already does
the reserving. Opening an older roster adds the `display` column and drops the
alias table in place; contributor refs written under the old id namespace are
left where they are, readable, and landable by a maintainer.