[#13102] Admin rejecting account request: give actual account in the email template

[Andy-W-Developer]

Fixes #13102

Outline of Solution Replaces accountRequestEmail with the requesters googleId and add some extra information. Gets the googleId via an admin search with accountRequestEmail as the searchKey. If no instructor account is found for that email, it will instead say "GOOGLEID NOT FOUND".

Regarding the fix: It's possible during development (not sure about live) to accidentally create multiple accounts if the googleId chosen is invalid, resulting in nothing after "TEAMMATES using your Google account:". The fix goes through all instructors with accountRequestEmail and uses the googleId of the last account with a non blank googleId.

[Andy-W-Developer]

Changed the missing googleId string from "GOOGLEID NOT FOUND" to "NO_GOOGLEID" as its shorter and the underscores makes it more obvious.

Added two tests for setting the existingAccount googleId when there are instructor accounts found, and when there are no instructor accounts found.