Support MO and math.SE questions

[StevenClontz]

To ensure that everything in our database hinges on some external form of peer review, we should support citations of https://mathoverflow.net/ and https://math.stackexchange.com/ questions. That way if one wants to assert something that's not in the literature, they can, provided they also pose the question/answer on the appropriate site. That way we can move quickly to add content and rely on those existing communities to keep us honest.

[StevenClontz]

I mocked up one here:

pi-base/data@341bd6a#diff-096238d0355b601572da74cc201d6a0a

So I'm suggesting mathse: for math.stackexchange and mo: for mathoverflow.

[StevenClontz]

Note that mathse: 3809662 should convert to the URL https://math.stackexchange.com/questions/3809662