Make teacher invitation email addresses case-insensitive (#562)

## Status

- Related to
RaspberryPiFoundation/digital-editor-issues#662
and
RaspberryPiFoundation/digital-editor-issues#663

## What's Changed?

- Modified email address comparison logic for teacher invitations to
make it case-insensitive as this was breaking for users if the email
address invited was not all downcased.

Author: loiswells97

app/controllers/api/teacher_invitations_controller.rb

@@ -29,7 +29,7 @@ def load_invitation
     end
 
     def ensure_invitation_email_matches_user_email
-      return if @invitation.email_address == current_user.email
+      return if @invitation.email_address.casecmp?(current_user.email)
 
       render json: { error: 'Invitation email does not match user email' }, status: :forbidden
     end

spec/features/teacher_invitations/accepting_an_invitation_spec.rb

@@ -155,6 +155,27 @@
         end
       end
 
+      context 'when inviation email and account email differ only in casing' do
+        let(:invitation_email) { user.email.upcase }
+
+        it 'responds 200 OK' do
+          put("/api/teacher_invitations/#{token}/accept", headers:)
+          expect(response).to have_http_status(:ok)
+        end
+
+        it 'gives the user the teacher role for the school to which they have been invited' do
+          put("/api/teacher_invitations/#{token}/accept", headers:)
+          expect(user).to be_school_teacher(school)
+        end
+
+        it 'sets the accepted_at timestamp on the invitation' do
+          freeze_time(with_usec: false) do
+            put("/api/teacher_invitations/#{token}/accept", headers:)
+            expect(invitation.reload.accepted_at).to eq(Time.current)
+          end
+        end
+      end
+
       context 'when invitation has already been accepted' do
         let(:original_accepted_at) { 1.week.ago.noon }