Also adds a custom rule to eslint. Since the recommended way of extending
eslint is to create plugins as standalone npm packages, the separate rule
is published as 'eslint-plugins-empty-returns'.
Fixes#8669.
This commit is easy to revert if we want to tone down errors
to warnings for the short term, while our codepath still does
proper handling for adding users when they come in messages.
This is mostly a pure extraction, but we needed to duplicate
a bit of setup from people.js, and I added an assertion
for warning about "No user_id provided".
Rohitt Vashishtha
Steve Howell