Fixes #93, editing multiple guides doesn't work
Ditch singleTask, use singleTop without history.

Using singleTask got us almost all the way there with what we wanted,
but there was some edge case bugs that kept popping up with back pressed.

For example:
* Navigate to guide A from "My Guides"
* View step 1
* Navigate to "My Guides" through the menu drawer
* View Guide B, step 1
* Tap "Back"
* Now the Guide A step list is showing which is wrong, Guide B list should be
  there.

Also, if viewing multiple guides, going back wouldn't ever go back to those
previous guides you viewed, which could be jarring for users.

Using singleTop with a combination of NO_HISTORY and overriding back to just
launch a new intent fixed these issues and get us the interaction we were going
for all along.