Scalar plugin download links should work in Colab

[wchargin]

Currently, the scalar plugin download links use <a href download>,
where the href points to a route on the TensorBoard backend. This
works fine locally, but on Colab requests to the backend are mediated by
a service worker, and the service worker does not appear to intercept
these requests. Consequently, downloads fail with a “network error”.

Proposed fix (to apply to Colab contexts only): When a run is selected
from the “run to download” dropdown menu, instead of updating the href
property of the links, send off two asynchronous fetch requests: one
to the CSV data and one to the JSON data. As these resolve, update the
hrefs to point to data URLs/blob URLs.

[nfelt]

FWIW, probably out of scope for now, but Stephan and I talked about moving the download data generation entirely to the client side so that there's no backend request required at all. This would match the download-as-SVG behavior and let run selection directly drive the CSV/JSON downloaded data selection instead of having to pick specific runs in the download dropdown.

Originally mentioned in #1446 (comment)

[wchargin]

Interesting. That’s an attractive suggestion: hiding behind a clear
“download” button rather than a dropdown selection is nicer, and it
avoids the problem of the blob data going stale as more data is loaded
from the backend. It also means that we could get rid of the “Data
download links” button—one fewer UI option to care about. And, of
course, the user doesn’t have to reselect the run that they’ve already
presumably selected.

For the sake of getting Colab unbroken, I’m somewhat inclined to keep
the scope limited, but if this is something that we want to prioritize
then I’d be more than happy to shift gears to it.

[nfelt]

Yeah I agree that I don't think we should work on this right now. But just FYI since it seems relevant.

[wchargin]

(Reopening: GitHub autoclosed this overzealously.)