Remove roles when block or delete users

After a user assigned as a budget admin deletes their account or gets blocked by
a moderator, the application throws an exception while loading the admin
investment index page.

As an erased user is not really deleted and neither its associated roles, the
application was failing when trying to sort and administration without a
username. In this case, the application was throwing an `ArgumentError:
comparison of NilClass with String failed` exception.

As a blocked user is not deleted or its roles, the application failed when trying
to access the user name through the delegation in the Administrator. In this
case, the application was throwing a `NoMethodError: undefined method `name' for
nil:NilClass` exception.

spec/models/user_spec.rb

@@ -538,6 +538,19 @@ describe User do
 
       expect(Identity.exists?(identity.id)).not_to be
     end
+
+    it "removes all user roles" do
+      user = create(:user)
+      [:administrator, :moderator, :manager, :sdg_manager, :valuator].each do |role|
+        create(role, user: user)
+      end
+
+      expect { user.erase }.to change { Administrator.count }.by(-1)
+                           .and change { Moderator.count }.by(-1)
+                           .and change { Manager.count }.by(-1)
+                           .and change { SDG::Manager.count }.by(-1)
+                           .and change { Valuator.count }.by(-1)
+    end
   end
 
   describe "#take_votes_from" do
@@ -749,6 +762,19 @@ describe User do
       expect(Legislation::Proposal.all).to eq [other_proposal]
       expect(Legislation::Proposal.with_hidden).to match_array [proposal, other_proposal]
     end
+
+    it "removes all user roles" do
+      user = create(:user)
+      [:administrator, :moderator, :manager, :sdg_manager, :valuator].each do |role|
+        create(role, user: user)
+      end
+
+      expect { user.block }.to change { Administrator.count }.by(-1)
+                           .and change { Moderator.count }.by(-1)
+                           .and change { Manager.count }.by(-1)
+                           .and change { SDG::Manager.count }.by(-1)
+                           .and change { Valuator.count }.by(-1)
+    end
   end
 
   describe "#full_restore" do