Implement "back" button functionality after the user has clicked pause

[amalnanavati]

As we observed in PR #30 (in service of issue #10 ), the question of what state the "Back" button should go to and in what circumstance is quite complicated. Hence, we broke that off into a dedicated issue.

This issue encompasses the following:

1. Make a table of all the reasons users would want to press "back" and how the robot should behave in each scenario. See this table as a solid start.
2. Informed by that, come up with a plan for where the Back button will go in each state/circumstance (or whether there should be multiple back buttons that the user can choose from).
3. Implement it.

[amalnanavati]

Closed with #41