docs: update lti_url for testing LTI 1.1/1.2

- update lti testing url in the README, as saltire has updated the test URLs.
- add a note in the README to use `norefresh` param, to avoid failures if using more than one
  same LTI xblocks in the same unit. Details can be viewed here: #379
- close #379.

README.rst

@@ -160,7 +160,11 @@ http://edx.readthedocs.io/projects/open-edx-building-and-running-a-course/en/lat
    as seen above).  Make a unit, select "Advanced", then "LTI Consumer".
 3. Click edit and fill in the following fields:
    ``LTI ID``: "test"
-   ``LTI URL``: "https://lti.tools/saltire/tp"
+   ``LTI URL``: "https://saltire.lti.app/tool"
+   **Note:** If you are using more than one same LTI xblocks in the same unit, 
+   please append the `norefresh` parameter to the LTI URL to avoid any 
+   potential failures. Then LTI URL will look like this: 
+   `https://saltire.lti.app/tool?norefresh`.
 4. Click save.  The unit should refresh and you should see "Passed" in the "Verification" field of
    the message tab in the LTI Tool Provider emulator.
 5. Click the "Publish" button.

lti_consumer/lti_xblock.py

@@ -646,7 +646,7 @@ def workbench_scenarios():
                         ask_to_send_email="False"
                         enable_processors="True"
                         launch_target="new_window"
-                        launch_url="https://lti.tools/saltire/tp" />
+                        launch_url="https://saltire.lti.app/tool?norefresh" />
 
                     <lti_consumer
                         display_name="LTI Consumer - IFrame"
@@ -656,7 +656,7 @@ def workbench_scenarios():
                         enable_processors="True"
                         description=""
                         launch_target="iframe"
-                        launch_url="https://lti.tools/saltire/tp" />
+                        launch_url="https://saltire.lti.app/tool?norefresh" />
                 </sequence_demo>
              '''),
         ]