Enable teachers to add (read config.xml uri) tools from the edu-apps.org LTI catalog

Simplify the process, for teachers, to add a tool from the edu-apps.org LTI catalog (which is publicly published for Instructor's Canvas teachers) to an External LTI activity.

After applying the attached patch, a Teacher could:

1. Browse the edu-apps.org catalog. ("edu apps catalog" screen capture) and Select a tool.
2. Copy its configuration xml uri ("copy tool's configuration url" screen capture)
3. Paste it into a new text input field "Tool configuration (XML) URL" (leftmost screen capture)
4. Click the "+" button (next to the "External tool type" field)
5. New window opens up with tool's configuration details (parsed) available on the proper fields. (2nd from the left, screen capture)
6. Update the relevant settings and click "Save" to go back into the LTI module settings page to finish setting up the tool.