Custom icons for third party auth login buttons

- Icon images can be uploaded from the django admin
- Test coverage improved

themes/stanford-style/lms/templates/register-form.html

@@ -26,7 +26,14 @@ from student.models import UserProfile
 
   % for enabled in provider.Registry.accepting_logins():
     ## Translators: provider_name is the name of an external, third-party user authentication service (like Google or LinkedIn).
-    <button type="submit" class="button button-primary button-${enabled.provider_id} register-${enabled.provider_id}" onclick="thirdPartySignin(event, '${pipeline_urls[enabled.provider_id]}');"><span class="icon fa ${enabled.icon_class}"></span>${_('Sign up with {provider_name}').format(provider_name=enabled.name)}</button>
+    <button type="submit" class="button button-primary button-${enabled.provider_id} register-${enabled.provider_id}" onclick="thirdPartySignin(event, '${pipeline_urls[enabled.provider_id]}');">
+        % if enabled.icon_class:
+        <span class="icon fa ${enabled.icon_class}" aria-hidden="true"></span>
+        % else:
+        <span class="icon" aria-hidden="true"><img class="icon-image" src="${enabled.icon_image.url}" alt="${enabled.name} icon" /></span>
+        % endif
+        ${_('Sign up with {provider_name}').format(provider_name=enabled.name)}
+    </button>
   % endfor
 
   </div>