Add ‘caseworking’ service permission

We want to add the option for a user to have a more restricted view of
Notify.

This should be opt in/out on a per service basis, because it will be
appropriate only for a minority of teams. It shouldn’t be the default
because most teams (those without a lot of users or those only using the
API) won’t need it.

So this commit adds it as a new service permission, which means we can
roll it out:
- in prototype form initially
- then as an opt-in service setting

app/models.py

@@ -239,6 +239,7 @@ EMAIL_AUTH = 'email_auth'
 LETTERS_AS_PDF = 'letters_as_pdf'
 PRECOMPILED_LETTER = 'precompiled_letter'
 UPLOAD_DOCUMENT = 'upload_document'
+CASEWORKING = 'caseworking'
 
 SERVICE_PERMISSION_TYPES = [
     EMAIL_TYPE,
@@ -251,6 +252,7 @@ SERVICE_PERMISSION_TYPES = [
     LETTERS_AS_PDF,
     PRECOMPILED_LETTER,
     UPLOAD_DOCUMENT,
+    CASEWORKING,
 ]