Update Epic widget to show placeholder value

Expose a new behavior in our generic sidebar_dropdown_widget. The "new" behavior is to add an intermediate step where Edit triggers a popover, which then triggers the original dropdown.

From the backend, we'll get the epic_is_set or has_epic key, which we can use in conjunction with epic to compute if the current user needs to see the placeholder.

Designs

#367302[CROSS_GROUP_ISSUES_ISSUE_VIEW.png]