Change scheme configurations to use 'Multi Select' instead of 'Select List'

Take, for example, the Notification Scheme.

Currently, if the user would like to configure which project role can receive notifications, they have to select them one-by-one (select one, apply the changes, then select the next one). In the event that there are a lot of project roles in an instance, it would be tedious and time-consuming to select the roles one-by-one.

Hence, it would be useful to be able to select multiple of the roles at the same time, which can be done via 'Multi Select' fields. This way, the user can apply the changes to multiple roles at once, or all of the roles at once, if they desire.