Pull due dates from the fields ghurlbot creates

[jyasskin]

Per @matatk, ghurlbot creates issues with due dates either as ISO dates (yyyy-mm-dd) in the issue title or in a Due: yyyy-mm-dd (more stuff) line in the first comment. If an issue has a date like that, it should count as triaged, and that date should be its SLO.

I have to check if title-based dates have a consistent-enough meaning to use for this. @matatk mentioned that they might sometimes be the issue creation date, which isn't useful. If they're a charter-review period phrased as start > end, that'll be useful.

I'll probably list these (along with milestone-based due dates (#2)) in a separate section from the 'urgent' and 'soon' sections. Particular groups probably won't have both kinds of issues.