Create a new setting that will enforce unique contact email addresses.

1. Create a new page, "General" under Settings > Company > Client Options
   • This will be the first, default page for that subsection
   • Add permissions for this page
2. Create a new setting, "Enforce Unique Contact Email Addresses" with options:
   • "-- None --", a blank value, that represents the current behavior, i.e., no enforcement of unique email addresses
   • "Primary Contacts", the value "primary", that enforces unique email addresses amongst all primary contacts for the company
   • "All Contacts", the value "all", that enforces unique email addresses amongst all contacts (e.g. primary, billing, other) for the company
   • A tooltip for this setting may also be useful, detailing this behavior succinctly
3. Update the documentation to include this setting on the company settings page
4. When adding or updating a contact (i.e. Contacts::add, Contacts::edit), perform a rule validation on the email that checks the value of the setting and performs a unique check on matching contacts such that the given email address is not used by any other contact (besides the current contact on edit).