* Make `SIGNUPS_DOMAINS_WHITELIST` override the `SIGNUPS_ALLOWED` setting.
Otherwise, a common pitfall is to set `SIGNUPS_DOMAINS_WHITELIST` without
realizing that `SIGNUPS_ALLOWED=false` must also be set.
* Whitespace is now accepted in `SIGNUPS_DOMAINS_WHITELIST`. That is,
`foo.com, bar.com` is now equivalent to `foo.com,bar.com`.
* Add validation on `SIGNUPS_DOMAINS_WHITELIST`. For example, `foo.com,`
is rejected as containing an empty token.
Ignore a missing `id` query param; it's unclear what this ID represents,
but it wasn't being used in the existing bitwarden_rs code, and no longer
seems to be sent in the latest versions of the official clients.