Add back button for when user is searching
Add back button for when user is searching

It's not intuitiv eenough for the user to know to clear out the search
to go back to the last topic. A back button makes sense here.

Test Plan:
- Do a search, click the back button, make sure it takes you to the last
  topic you were at.

Khan/khan-academy-fxos,Khan/khan-academy-fxos