Tasks: Easier editing

Problem: Editing tasks is too slow/difficult. Changing the task status is easy, however editing the assignee and due date takes too many clicks.

Possible solution: Add inline-editing for assignee and due date on task/show.

Prior art

Linear has a nice UX for this:

GitHub also has inline editing in the item side panel:

Steps

1. Update status menu dropdown to new style
2. Add assignee dropdown menu
3. Add due date dropdown menu
4. Simplify task/:id/edit form by removing status/assignee/due-date fields
5. Release!