Feature request: "Select all" checkbox

[gabrc52]

A button to select or deselect all categories would be nice.

Example user flow: User wants to see all free food today/tomorrow. This would require deselecting every category except for food, which is 9 clicks. The next day, they want to see all dormspam. This would require 9 clicks again, to select every category.

I could give it a shot to implement / could let me familiarize with y'alls codebase.