The user_selector is unintuitive on mobile devices

The user_selector is unintuitive on a mobile device, such as an iPad. On initial load the "Current users" and "Potential users" drop down lists show the text "0 items". This can be interpreted as meaning that 0 items were found. When in fact 0 items have been selected.

A suggestion is to change this text so that it says "0 items selected" instead.

Steps to replicate:
1. Access Moodle using an iPad
2. Login as an admin
3. Navigate to Site Administration -> Users -> Accounts -> Cohorts
4. Add a new cohort
5. Touch the "Assign" users icon
6. View the cohort members page

Expected Outcome:
The "Current users" and "Potential users" drop down lists show "0 items selected".

Actual Outcome:
The "Current users" and "Potential users" drop down lists show "0 items".