Make deletion confirmation banner messages consistent across our app

Also introduce a way to provide context to a banner / flash message
that will be displayed in plain font style.

app/main/views/api_keys.py

@@ -130,9 +130,12 @@ def create_api_key(service_id):
 def revoke_api_key(service_id, key_id):
     key_name = current_service.get_api_key(key_id)['name']
     if request.method == 'GET':
+        flash([
+            "Are you sure you want to revoke this API key?",
+            "‘{}’ will no longer let you connect to GOV.UK Notify.".format(key_name)
+        ], 'revoke')
         return render_template(
             'views/api/keys.html',
-            revoke_key=key_name,
         )
     elif request.method == 'POST':
         api_key_api_client.revoke_api_key(service_id=service_id, key_id=key_id)