Move Airtable build into custom function

[thchia]

Problem: Without an Airtable API Key, it's not possible to run the project locally. This is kind of against the philosophy where anyone should be able to contribute to this project (correct me if I am wrong).

Proposed solution: We can move the Airtable fetching logic into a custom function, and replace the Airtable plugin with a custom plugin that queries our function instead. Then the Airtable API key is only needed to build the function code, which is not strictly necessary if you are not working on any of the auth/RSVP stuff.