GitHub link disappears intermittently in 3.x

Created by: keegancsmith

From customer:

Since the 3.x upgrade we've been seeing the :github: link disappear intermittently. Sometimes reloading the page fixes it.

It's almost always happening, I don't see a discernable pattern in the times that it does show up

I first noticed this when testing 3.x out on our test instance, and at the same time it worked properly on our production instance (running 2.x), so it seems to be related to the upgrade

More info (possibly separate issue):

More info: we are seeing the following under the "Mirroring" details for the repos where the link isn't showing up. In some cases they are being refreshed properly, in some cases they are showing as very out of date

https://sourcegraph.slack.com/archives/GDS3PJ8GK/p1551726810000500

Reported by https://app.hubspot.com/contacts/2762526/company/561806411