Make sure we're measuring the reachability of our gettor distribution methods

Right now users can download gettor links from 4 different locations:

• GitLab repository
• Github release
• Internet Archive
• Google Drive

We should make sure we use existing means for checking reachability (e.g., OONI) for keeping track of whether these locations are reachable. If there is a region where none are reachable, we need to figure out new distribution aveneues.