WISH: Disable 'Bug tracker' fallback

[HenrikBengtsson]

It looks like R-universe falls back to using the issue tracker of the repo where the packages.json file lives, if the package DESCRIPTION file does not have a BugReports field.

If you just provide an "umbrella" R-universe repository, where you just "relay" packages owned by others, you don't want people to start reporting issues to you. For instance, I'd like a way to not show:

Bug tracker: https://github.com/cranhaven/cranhaven.r-universe.dev/issues 2 issues

on https://cranhaven.r-universe.dev/validateRS#.