Major Description
Today, webadmin relies on JWT configuration for the JMAP protocol.
This brings concerns, as the tenant are distinct (users vs admins), and the token issuers are likely distinct.
The compromission of a webmail service would today easily grant access to the webadmin APIs.
As such it is desirable to be able to specify distinct keys for both protocols.
In order to avoid breaking changes, if the webadmin JWT public key is unspecified, we should fallback to the JMAP one.