UX: Easier to select multiple task performers

Tasks often have multiple task performer candidates. Additionally, the same performers are usually needed for tasks in the same lane. We've made it easier to select multiple performers by using a different multiselect modal. This modal remains open until you click out of it, making it easier to select multiple performers. Additionally, when performers have been added to any task in a lane, all other tasks in the lane will display those performers at the top of the search options.