Sort PosterSwiper to sort by Priority, Involvements, and then Expiration Dates

[Jshmill]

Already solved, just documenting:

Backend here: #1252

Frontend here: gordon-cs/gordon-360-ui#2747

Right now the Swiper for Posters on the home page sorts by Priority first (for CEC), and then by Expiration date. This change makes it so that priority comes first, then all the clubs that that specific user is involved in, and then finally expiration date, making the order more personalized.

AS OF RIGHT NOW, MERGING EITHER BACKEND OR FRONTEND WITHOUT THE OTHER BREAKS POSTERS!!! Im fixing it right now but if I can't, just dont merge one without the other.