Add skills to tasks

[begedin]

Problem

We want users to be able to add and remove skills to/from tasks

Subtasks

• Assigned skills should be visible on task card - Add skills to tasks #1045
• Add/remove UI should be present on the task page - Add skills to tasks #1045
• Add/remove UI should be present on the task new page? - Add task skill UI to the new task page #1046
• Add/remove UI should only be usable by project contributors or higher - Only task authors as well as project contributors or higher can edit task skills. #1043

I think the last one, considering how much harder it is to deal with, might be OK to punt for a later time. I'm not really sure, but I think it's worth discussing.