Prototype v2: Custom activity authoring

[carolynzhang18]

Notion ticket link

Ticket Name

Implementation description

• Attempt at integrating Aathi's grid layout prototype with our backend/DB.
• Prototype v2 of "custom activity type".
• Some schema changes - subject to change depending on finalized designs.

Steps to test

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