Allow to search on active membership

Currently, when defining status rules for memberships, you must set whether a specific rule corresponds to being a member or not (the column is_current_member in the table membership_status). The thing is, nowhere can be searched on just this column, which is in fact much more practical than enumerating all wanted states for a membership when searching for active members.
So I've created a small patch that allows to search this column too. Now before I create a PR, any suggestions on the name of the field in the members search form template? Currently I call it "Member?", but maybe "Active member?" would be more appropriate