Prefer unclosed PRs when linking from milestone issues

[severinrudie]

Feature request:
In milestone view -> multiple PRs fix an issue -> all but one of those PRs has been closed
Requested: link to only open PR
Actual: Display "PR > 1"

For an example, see milestone: https://github.com/mozilla-mobile/firefox-tv/milestone/16.
Issue: mozilla-mobile/firefox-tv#1159
PR1 (open): mozilla-mobile/firefox-tv#1252
PR2 (closed): mozilla-mobile/firefox-tv#1160

[mcomella]

Consider some of our users interpret "Has a PR" as "almost done", I think this makes sense. I wish there was a way to indicate these are "open PRs" but we don't have much space.

We currently don't store open/closed state for PRs so this would require:

• Updating DB schema to include a list of open PRs (probably not in the issue -> PR mapping so we can use this list to look up open PRs)
• When scraping PR page, update is open information
• Refresh open PRs list when scraping open PRs (this will be partially broken until Get more than one page of GitHub API results #25 is fixed but this won't really affect our repos where we don't usually have more than 20 PRs open)
• Expose in UI

[mcomella]

@baron-severin How useful would this be to you?

It adds a lot of complexity to the code and update algorithms so I'm hesitant to add it if it's a small win.

[severinrudie]

@mcomella It's a nice to have. Given the complexity laid out above, I think closing the issue is reasonable.