Add functionality to clear the selected guest

[hfaerber]

Need to determine when (on what event) to clear the selected guest on the manager page.

Clear guest on the display and clear guest by setting selectedUser variable back to null so no rooms can be accidentally booked for the previous guest.

[hfaerber]

I could disable the search input field until a 'Search New Guest' button is clicked which would clear the previous guest from the DOM and reset the selectedUser variable to null and re-enable the search input field.