Set default_label attribute on weight issue filter.
The label for the "Weight" filter in the issues list is darker than the other filters.
This MR sets the default_label attribute to 'Weight', so that the styles for default labels are applied to "Weight". This is the convention for other filter dropdowns.
I'm guessing it regressed here 8961364c. The default_label was not set when the weight filter was originally transitioned to the new dropdown (https://gitlab.com/gitlab-org/gitlab-ee/commit/b0639b1fdafd0cf48f1fcdaa8047f7414010e643). I'm guessing some styles were unintentionally cascading to give this the correct look, and were changed.
After the fix:
