Login hooks.

Adds `Accounts.validateLoginAttempt`, `Accounts.onLogin`, and
`Accounts.onLoginFailure`.

The process for logging in a user is consolidated in accounts-base,
which is now the only package which directly accesses login tokens in
the database.

All login methods now go through `Accounts._loginMethod`, which
ensures that exceptions are captured and login hooks are called in all
cases.

The callback hook implementation code from livedata is extracted into
an internal `callback-hook` package, where it can be used by accounts.