api endpoint for program page config

[shanbady]

What are the relevant tickets?

Closes #3711

Description (What does it do?)

This PR has changes that integrate wagtail built-in api endpoints so we can retrieve content used in program letters from mit-open.

How can this be tested?

1. Checkout this branch.
2. If you dont already have some mock data, create a ProgramPage via /cms - make sure it is not sitting in the 'root' (should be a child of the "Micromasters Home" page. Fill out the required fields in addition to the fields starting with "program_letter_*"
3. Save and publish the page
4. navigate to the wagtail endpoint and specify the pagetype and wildcard for the fields http://localhost:8079/api/v0/wagtail/pages/?type=cms.ProgramPage&fields=*
5. See that all the program letter fields are shown (inckuding the list of signatories) in addition to links for the signature images

[annagav]

LGTM 👍