Browser ext doesn't work w/o "Enable on this domain", but this isn't obvious to users

Created by: dadlerj

After a long discussion with an Engineer at Codecov trying to debug why they couldn't see anything from Sourcegraph on GitLab, we finally discovered they hadn't yet "enabled" it on that domain (by default, it only works on github.com I believe).

Can we either:

1. make it work by default on gitlab.com and other major code hosts
2. add a badge/alert to the ext icon if it's not enabled (like how other browser extensions do this)

3. simply add a button for this in the actual extension menu?

@francisschmaltz you might have design ideas