Challenge achievements need a way to specify which week they apply to

Spin-off of https://github.com/mozilla/voice-web/issues/2431.

Upon adding the Week 1 prizes:

INSERT INTO earn (achievement_id, team_id) VALUES(3, 1);
INSERT INTO earn (achievement_id, team_id) VALUES(4, 3);
INSERT INTO earn (achievement_id, team_id) VALUES(5, 2);

This happened:

Observed: Week 1 results ended up in Week 2, and Week 1 shows a three-way tie for first.

Expected: Week 2 should remain blank. Week 1 should display what’s currently in Week 2.

[phirework]

Hmm I think for non-weekly achievements we’ll still need the date that it was earned, but it might be worth considering adding an extra column to the achievements table that defines which week’s points it’s meant to count for, with NULL meaning “any week”?

[phirework]

@rileyjshaw Yep I can deal with that.