zilencer: Tighten type annotations of views.
zilencer: Tighten type annotations of views.

`remote_server_path` allows us to get rid of all the `validate_entity`
calls in `zilencer.views` and remove all the `Union` type annotations
in the signatures of the authenticated view functions.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

rht/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip