Add an option for Assignee/Reporter permissions to be applied to User Picker custom fields

Problem Definition:

User Picker custom fields lack the same level of protections for the Assignee and Reporter fields - see the 9.11 release notes:

• Assignee: "This picker will now show all users that have the ** Assignable user permission in the project."
• Reporter: "The picker will show users with Browse projects permission in the project."

Suggestion: Add an option that allows for these same permissions be applied to Custom Fields using User Pickers. At present there is the option to filter browsing user picker fields to specific groups or roles, but this represents a duplication of effort and additional group management tasks for the customer if they already have the above configured.