RFC: PRs should include an assignee

[orta]

Proposal:

Warn that a PR that is not classed as a WIP needs an assignee.

Reasoning

This rule is in most Dangerfiles in our org, and is generally a pattern everyone uses due to our
slack bot integration. This formalises it to all repos.

Exceptions:

This is probably worth having as a warning for a long time, then moving to a failure once we're all used to the pattern - like 6 months kind of time.