Merge branch 'heavy-weights' into 'master'

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.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/1094

See merge request !794