Replace hard-coded URLs pointing to upstream repos with variables that can point to our repos

[DeeDeeG]

There are some more hard-coded URLs pointing to github.com/atom/atom and github.com/atom/atom-nightly-releases. We should replace these with REPO_OWNER, MAIN_REPO and NIGHTLY_RELEASE_REPO, the same way that it's being handed in/as discussed in #78.

Specifically, for example:

const MAIN_REPO = process.env.MAIN_REPO || 'atom'

[DeeDeeG]

easy way to find many of these is to search for things like atom/atom or JS strings exactly matching the project/repo name 'atom' or even better 'atom-nightly-releases', although there are some other hard-coded usages that don't match those search patterns.

Some of them are less important than others, or we can live with the hard-coded version. For example, apparently we pull the Atom stable/beta/nightly/dev icons from upstream at the moment? But we haven't used a custom icon yet, so it doesn't make a difference.

There's obviously Atom's "Find in Project", or if you prefer grep, you can do git grep

example: git grep \'atom-nightly-releases\' -- script/vsts