FEATURE: Warn if invited user cannot see topic (#13548)

Users can invite people to topics from secured category, but they will
not be redirected to the topic after signing up unless they have the
permissions to view the topic. This commit shows a warning when invite
is saved if the topic is in a secured category and none of the invite
groups are allowed to see it.

spec/models/invite_spec.rb

@@ -404,4 +404,27 @@ describe Invite do
       expect(invite.invalidated_at).to be_nil
     end
   end
+
+  describe '#warnings' do
+    fab!(:admin) { Fabricate(:admin) }
+    fab!(:invite) { Fabricate(:invite) }
+    fab!(:group) { Fabricate(:group) }
+    fab!(:secured_category) do
+      secured_category = Fabricate(:category)
+      secured_category.permissions = { group.name => :full }
+      secured_category.save!
+      secured_category
+    end
+
+    it 'does not return any warnings for simple invites' do
+      expect(invite.warnings(admin.guardian)).to be_blank
+    end
+
+    it 'returns a warning if topic is private' do
+      topic = Fabricate(:topic, category: secured_category)
+      TopicInvite.create!(topic: topic, invite: invite)
+
+      expect(invite.warnings(admin.guardian)).to contain_exactly(I18n.t("invite.requires_groups", groups: group.name))
+    end
+  end
 end