Fix negative clean time

[jaykim1213]

Does this PR close any open issues?

Closes #962

Give a longer description of what this PR addresses and why it's needed

• Fix negative clean time - return -1 if there is negative clean time (browser events missing/buggy)
• Show an icon in the stats that shows that some data points were excluded from the duration calculation.

Are there any additional TODOs before this PR is ready to go?

TODOs:

• Update relevant documentation

[github-actions]

A preview of 3d42f01 is uploaded and can be seen here:

✨ https://revisit.dev/study/PR979 ✨

Changes may take a few minutes to propagate.

[JackWilb]

Let's check why this happens? The logic seems to imply negative values are impossible

[JackWilb]

Calculate as -1 if the cleaned time is wrong because the browser events are missing/buggy.

Show an icon in the stats that shows that some data points were excluded from the duration calculation.

Add a note in the documentation that says if the cleaned time is -1 that there was some problem in computing that value, probably that the browser didn't give valid events (i.e. missing a visibility change)