Add discriminator for lesson page and activity page

[carolynzhang18]

Notion ticket link

Ticket Name

Implementation description

• Added discriminators for LessonPage vs. ActivityPage to accommodate PDFs for lessons (see coursepage.mgmodel.ts)

Steps to test

1. To test, what I did was import LessonPageModel and ActivityPageModel into server.ts, write a function that does [Lesson|Activity]PageModel.create({ <data> }), and then call this function in the server.listen callback. This should create a new document in the coursepages model in MongoDB.

What should reviewers focus on?

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR

[ji-mmyliu]

LGTM 🚀

Here's what my Mongo looked like after testing: