Add disable send policy (#1130)

* Add Disable Send policy

* Test DisableSend policy

* PR Review

* Update tests for using CurrentContext

This required making an interface for CurrentContext and mocking out
the members used. The interface can be expanded as needed for tests.

I moved CurrentContext to a folder, which changes the namespace
and causes a lot of file touches, but most are just adding a reference

* Fix failing test

* Update exemption to include all exempt users

* Move all CurrentContext usages to ICurrentContext

* PR review. Match messaging with Web

src/Core/Enums/PolicyType.cs

@@ -8,5 +8,6 @@
         SingleOrg = 3,
         RequireSso = 4,
         PersonalOwnership = 5,
+        DisableSend = 6,
     }
 }