Used problem icon in vector draw xblock

[replaceafill]

This fixes xblock-vectordraw/issues/7.

Testing instructions:

1. Set up a devstack.

2. Install the xblock on this branch into the edxapp Python virtualenv.

   # In the same directory as your Vagrantfile:
   vagrant ssh -- sudo -u edxapp /edx/bin/pip.edxapp install -e git+https://github.com/replaceafill/xblock-vectordraw.git@issue_7#egg=xblock-vectordraw

3. Run Studio and LMS

4. Navigate to a course, and then "Settings > Advanced Settings".

5. Update the "Advanced Module List" field to contain ["vectordraw"].

6. Click "Save Changes".

7. Navigate to the outline and create a new Section, Subsection, and Unit. You will be redirected to the Unit authoring view.

8. Select "Advanced > Vector Drawing" to create a new Vector Drawing problem.

9. Click "Preview" and observe the problem icon in the LMS sequence navigation bar.

[mtyaka]

Thanks @replaceafill!

:+1:

• [x] I tested this: I confirmed that without this patch, vectordraw units use the default ("other") icon in edX LMS. I checked out this branch and confirmed that it makes LMS use the "problem" icon for vectordraw blocks.
• [x] I read through the code
• [x] I checked for accessibility issues
• [ ] Includes documentation - this is a minor change/bugfix, documentation not required.