Autogenerate owners and teams

[nikomatsakis]

This auto-populates the metadata on goals with lists of the teams involved and the owners of tasks. The goal is both to be more DRY and to to surface up the folks doing the work so that they are shown prominently, and not just the point-of-contact.

r? @lqd

Rendered

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[nikomatsakis]

Example:

[lqd]

impl/result look good to me.