We're seeing errors in logs due to some sites setting the reserved_usernames setting to nil. This is causing multiple use cases upstream of User#reserved_username? to error out.
This commit changes from using the raw #reserved_usernames to using the #reserved_usernames_map helper which exists on list-type site settings. It returns an empty array if the raw value is nil or empty string.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
