This adds the current_user_has_reacted_to_emoji() helper.
This new helper is easier to use and slightly more efficient
than calling get_user_list_for_message_reaction() and then
indexOf().
This also replaces one call to get_user_list_for_message_reaction()
with a list of user_ids that we already had locally.
The node tests were improved a bit here, including a minor
whitespace fixup.
If we get reactions for deactivated users, or otherwise missing
users, we only issue a blueslip warning now. The function
get_message_reactions() was indirectly causing blueslip errors
before this fix, but we can downgrade to warnings now that this
function has better unit tests around it.
We eventually want to track deactivated users on the client.
Fixes#4289
Steve Howell
Harshit Bansal