create a backend for displaying mutations

[haoyxc]

Add mutation backend so that the frontend arrows are functional!

changes include:

• start graph at beginning (0 mutations applied)
• generalize getting mutations so that you can request any given mutation
• the mutation function is in Utility. it takes in the original data and the current data. "previous" will start at the beginning while "next" applies mutations from the current graph.
• add tests for the mutation function
• if the user is at 0 and goes back, they get an alert. if they're at the last mutation and go to next, they also get an alert (can change to something else in a later PR, I believe Hao brought up toast once!)

in action: capture.zip

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to determine that you authored the commits in this PR. Maybe you used a different email address in the git commits than was used to sign the CLA? If someone else authored these commits, then please add them to this pull request and have them confirm that they're okay with them being contributed to Google. If there are co-authors, make sure they're formatted properly.

In order to pass this check, please resolve this problem and then comment@googlebot I fixed it... If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.