notebooks: enable stopword filtering in compute block

Created by: rvantonder

Stacked on https://github.com/sourcegraph/sourcegraph/pull/32647.

This makes stopword functionality work again. I removed it when introducing the app initially because I had a very large list. I'm using a much smaller list from the R package now. It's far too useful to not include--helps finding topics in commit messages:

https://user-images.githubusercontent.com/888624/158508370-3386ad18-c66b-4dd6-9b8b-78b5270e6f37.mp4

And also to not match on words in comments when you want to identify common code conventions (like, if X should match X variables in code, not someone who wrote a comment // if the thing).

Test plan

Manually tested for experimental feature