Models and migration for new models. Also update the admins.
Initial API for lab in xblock.
Basic quizblocks and problems API endpoints
More complete labs data in response
initial effort on editing problems in studio (for content admin)
Modify LabProxy, only need the lab and unit_id field for uniqueness

Also add functions to copy quizblocks and problems for lab proxy.
LabProxy API endpoints to list and create.
Enable LabProxy admin
QuizBlock.slug shouldn't be unique.
Allow edit LabProxy if lab_proxy_id is passed
Return lab in LabProxy API response
API updates for problems (and editing problems)

- add serializer to problem's detail view
- add some properties in labs to have unified source of data
Convert xml to markdown on save, and add content_html property.
Add answer field to Problem for each checking

Also add function to fetch answer from xml, and generate the answer
field using that function.
Model to store user's answer to problems.
API endpoint to answer problem
Create quiz block view
Create problem view
Create problem form and view
Function to convert markdown to xml.