Priority multiplier

From this ticket (https://github.com/HabitRPG/habitrpg/issues/3556) and this trello page (https://trello.com/c/4C8w1z5h/17-task-difficulty-settings-v2-priority-multiplier)

Additional buttons for difficulty settings under the priority settings in the advanced options.
Probably shouldn't have put task.weight update in ng-click, but didn't really know how to use $watch for class field, and didn't know if this would take a lot of resources.
(in all button's)