[CORL-409] Prevent users from ignoring staff members (#2355)

* Throw error if user tries to ignore a staff member

Throws a UserCannotBeIgnoredError if a user tries to ignore
a user who is a staff member. A staff member in this case is
considered anyone who has a role of staff, moderator, or admin.

CORL-409

* Prevent users from ignoring staff in the user info popover

Creates the staff roles in a constant next to the user model.
Uses this to add a computed property to the user resolver.

CORL-409

* Remove unnecessary async declaration from userIsStaff helper function

CORL-409

* Specify ignoreable on users in client test fixtures

Allows the tests to pass for the required computed property
of ignoreable that is computed by whether a user is a staff
member or not.

CORL-409

* Update more fixtures with ignoreable property on mocked users/commenters

CORL-409

* Consolidate ignore-able calculation into re-usable helper methods

Re-use the logic for whether a role is a staff member to
clearly define when a user is ignore-able or not across the
business logic.

CORL-409

* Set the ignoreable optimisticResponse on comment mutations

We have set the ignoreable value in the graphQL schema, so now
the optimisticResponses are looking for a default value to use
until the data result arrives. Put to false since the ignoreable
value is set on our author, we likely don't want to ignore ourselves.

CORL-409

src/core/client/stream/test/fixtures.ts

@@ -104,21 +104,25 @@ export const commenters = createFixtures<GQLUser>(
       id: "user-0",
       username: "Markus",
       role: GQLUSER_ROLE.COMMENTER,
+      ignoreable: true,
     },
     {
       id: "user-1",
       username: "Lukas",
       role: GQLUSER_ROLE.COMMENTER,
+      ignoreable: true,
     },
     {
       id: "user-2",
       username: "Isabelle",
       role: GQLUSER_ROLE.COMMENTER,
+      ignoreable: true,
     },
     {
       id: "user-3",
       username: "Markus",
       role: GQLUSER_ROLE.COMMENTER,
+      ignoreable: true,
     },
   ],
   baseUser
@@ -354,6 +358,7 @@ export const moderators = createFixtures<GQLUser>(
       id: "me-as-moderator",
       username: "Moderator",
       role: GQLUSER_ROLE.MODERATOR,
+      ignoreable: false,
     },
   ],
   baseUser