PLT-3762 - Add API call to get a user by their username (#4611)

* PLT-3762 - Add API call to get a user by their username

* fix lint

* update rote

* update per code review

* update per code review

* remove first/last name is not used in this test

webapp/tests/client_user.test.jsx

@@ -36,6 +36,21 @@ describe('Client.User', function() {
         });
     });
 
+    it('getByUsername', function(done) {
+        TestHelper.initBasic(() => {
+            TestHelper.basicClient().getByUsername(
+                TestHelper.basicUser().username,
+                function(data) {
+                    assert.equal(data.username, TestHelper.basicUser().username);
+                    done();
+                },
+                function(err) {
+                    done(new Error(err.message));
+                }
+            );
+        });
+    });
+
     it('getInitialLoad', function(done) {
         TestHelper.initBasic(() => {
             TestHelper.basicClient().getInitialLoad(