Implement "Undo" task option

[cafootitt]

After a user completes a task in the list, we need to give them an option to "Undo" that task, in case they made a mistake. This is especially relevant for the "Looks good" option, since that's relatively easy to mistakenly complete.

Workflow we need to support:

• [x] In the "Checked" task list, a user can tap on any of the completed tasks. Mockup for reference: https://marvelapp.com/prototype/1296hh24/screen/72966670

• [x] A dialog appears in this format: https://lh3.googleusercontent.com/zRa10al2A-xM-BXmf-RTeLZO38eUcCQ364zVEywSAezPGO40ln7_DF954UL8Z-swkK-yNzMJuhkdWWK4_Y3SJVzEZXwexkQJSsSPgPY=w1064-v0

• [x] The dialog title says Undo "looks good" on [product_name]? or Undo "flag problem" on [product_name]? (depending on whether they completed the task by either checking the looks good option or the flag problem form submission). Product_name is replaced with the name of the item that was checked.

• [x] The dialog text will read This will undo your assessment on this item.

• [x] The dialog button options will be Cancel or Undo. Tapping on Cancel or anywhere outside the dialog will cancel the action; tapping on Undo option will close the dialog, and the task will be moved back to the top of the list and appear in its undone state. The user will be able to tap on the task to complete it again.

[cafootitt]

V0.0.1 build date of 10 Dec 2020 - lgtm - good work!