We should make `get_by_user_id` type-safe in the sense that it's caller
should not expect undefined values and hence it's caller should not
need to validate the user ids.
To do this I extracted a method `maybe_get_user_by_id` which is
type-unsafe version of this function which will allow undefined values
and will behave exactly the same as the previous version. So the callers
which expects the undefined values from this function should use this
new function `maybe_get_user_by_id`.
Probably about half of these callers are implicitly iterating through
all users in the people.js data set and thus by construction should
never fail, but it's simpler to just convert all existing callers than
do that audit.
This lets us simplify the long-ish ‘../../static/js’ paths, and will
remove the need for the ‘zrequire’ wrapper.
Signed-off-by: Anders Kaseorg <anders@zulip.com>