feat(app): create landing page generation UI

[nicholaschiang]

We need to create an onboarding UI that will enable schools to setup their student support landing pages in under five minutes. Such a UI should include pages to:

1. Select their subdomain (e.g. gunn.tutorbook.app for Gunn High School).
2. Add a description for their school (it would be nice if we could support formatting this too... something like Slack's markdown messages stuff).
3. Configure their website rules and location data. They should also select (or create) their school district here as well.

This should create two (or possibly three) Firestore documents:

1. The tutoring location in the locations collection.
2. The website configuration in the websites collection.
3. Possibly a district or access configuration in the access collection.

This should emulate Slack's onboarding process for choosing a workspace URL, etc.

[nicholaschiang]

Ok, I built out the pre-login UI but we still need to work on:

1. Actually creating the necessary Firestore documents.
2. Only doing so once the user has signed in (so passing the data
   collected back to the app after the Google Sign-In redirect succeeds).
3. Guiding the user through specifying location hours and service hour
   rounding, creating users and appointments, etc (this will most likely be
   done with Intercom once we get on their free COVID-19 plan).

Current pre-login onboarding UI looks something like Slack's (ultimate goal would be to render a preview of their landing page on the right; kind of like Slack does):

Let me know @budiantoip @davidrlunag @marylicious if you're up to helping out with any of the above! 👍