Enable batch actions

[ginatrapani]

In the task list view, as it is in the Android app, you should be able to select a single task or multiple tasks by tapping them, and get access to a menu of possible actions to perform on those tasks.

This means we get rid of the Task Details screen entirely, and we only have the list view, and Add/Update Task.

Action menus on for selected task(s):

• Single task: Complete (or Undo Complete if already complete), Edit, Prioritize, Delete
• Multiple tasks: Complete (if all incomplete, or Undo complete if all complete), Prioritize, Delete

This issue supersedes #96 and #121