Add API call to get a user by their email address (#4884)

* Add API call to get a user by their email address

* update per review

webapp/tests/client_user.test.jsx

@@ -51,6 +51,21 @@ describe('Client.User', function() {
         });
     });
 
+    it('getByEmail', function(done) {
+        TestHelper.initBasic(() => {
+            TestHelper.basicClient().getByEmail(
+                TestHelper.basicUser().email,
+                function(data) {
+                    assert.equal(data.email, TestHelper.basicUser().email);
+                    done();
+                },
+                function(err) {
+                    done(new Error(err.message));
+                }
+            );
+        });
+    });
+
     it('getInitialLoad', function(done) {
         TestHelper.initBasic(() => {
             TestHelper.basicClient().getInitialLoad(