Feat/account creation hook

[joshwanf]

Resolves #273

Create an account action follows this flow:

1. "Sign up" or "Register with Google" is clicked.
2. Firebase Auth handles account creation and resolves promise.
3. .then() calls backend's POST /api/account-hooks with an array of side-effect hooks to run. If Google sign in was used, .then() checks for userCredential.operationType as link or reauthenticate.
4. Web app layout <ProtectedRoute> checks for an isProvisioningAccount signal to sync loading AFTER backend finishes.

Currently, AccountHookAction is an enum of only "add example screener." A Map connects an AccountHookAction to a function, so new hooks can be added.