models: Replace is_guest and is_realm_admin with UserProfile.role.

This new data model will be more extensible for future work on
features like a primary administrator.

zerver/data_import/import_util.py

@@ -63,8 +63,7 @@ def build_user_profile(avatar_source: str,
                        full_name: str,
                        id: int,
                        is_active: bool,
-                       is_realm_admin: bool,
-                       is_guest: bool,
+                       role: int,
                        is_mirror_dummy: bool,
                        realm_id: int,
                        short_name: str,
@@ -79,8 +78,7 @@ def build_user_profile(avatar_source: str,
         id=id,
         is_mirror_dummy=is_mirror_dummy,
         is_active=is_active,
-        is_realm_admin=is_realm_admin,
-        is_guest=is_guest,
+        role=role,
         pointer=pointer,
         realm_id=realm_id,
         short_name=short_name,