Reading: Update GoogleSheetsFetcher to recur into folders and batch calls

[kevinrobinson]

Who is this PR for?

Somerville K5 students, reading specialists, literacy coaches

What problem does this PR fix?

Previous PRs like #2619 #2620 #2621 worked out the kinks running the importer task over individual schools, but running it over the whole district ran into a few issues related to fetching data. First, #get_tabs_from_folder assumed that all files within a folder were sheets, but turns out this isn't always the case. Second, it seems that the in parent query in Google Drive searches only within the immediate "children" of a folder and not recursively which I think I was assuming. Third, when updating to search recursively, we hit API request quotas.

What does this PR do?

1. Updates the methods making API calls to explicitly limit results to files that are sheets or folders.
2. Adds a recursive: true option to #get_tabs_from_folder.
3. Uses a batch method to get values from all sheets in a spreadsheet in one request.
4. Handle the special case of the API returning nil when calling #values on a newly added sheet.
5. Add optional logging that was useful in debugging.

Does this PR use tests to help verify we can deploy these changes quickly and confidently?

• Manual testing made more sense here

[kevinrobinson]

@edavidsonsawyer This fetcher class is getting some good mileage! :) I'm going to merge this now, but it'd be awesome if you could review and share any comments or feedback whenever you have a chance.