Selectively disallow opt-in based on patron's home library
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
A new org unit setting has been added which enables a library to prevent their
patrons from being opted in at other libraries.

For example, consider the following org unit hierarchy:

       Org Units          Depth

          CONS              0
           |
      +-----+-----+
      |           |
     SYS1        SYS2       1
      |           |
   +--+--+     +--+--+
   |     |     |     |
  BR1   BR2   BR3   BR4     2

Suppose that SYS1 wishes to prevent its patrons from being opted in at SYS2.
To accomplish this, it sets the value of the "Restrict patron opt-in to home
library and related orgs at specified depth" setting to 1, meaning that patrons
at SYS1 libraries at or below that depth in the org tree cannot be opted in by
libraries outside that part of the org tree.  Thus, BR1 patrons can be opted in
at BR2, but not at BR3 or BR4.

(This setting is distinct from the "Patron Opt-In Boundary" setting, which
merely determines the depth at which Evergreen prompts for the patron to opt
in.)

New Library Setting
+++++++++++++++++++
  * Restrict patron opt-in to home library and related orgs at specified depth (org.restrict_opt_to_depth)