User tokens cannot be edited

It is not possible to edit a user token.
You can generate and delete a token, but can not edit its value.

## Expected Behavior
There are scenarios where it is useful to use a token that
has been created outside of graylog.
Take the `graylog-sidecar` user token for example:
You want to migrate to a new graylog server, but want to keep the existing
token, because otherwise you'd have to update all existing Sidecars to a new token.