BIG: Contact overhaul

This one's a doozy.

This PR substantially overhauls several portions of the API to improve stability and clean up technical debt.

A snapshot of notable changes:

Contact migration

• Replace overgrown Member model and API with Contact model and API
• Support for multiple email addresses, with an editable primary email address
• Move from "First Name" "Last Name" columns to "Full Name" "Display Name" to improve cultural accommodation of different naming schemes
• Methods of contact (email addresses, phone numbers, postal addresses) now include fine-grained consent tracking which is opt-in by default.

Tags and Custom Fields

• Overhaul the Tag API to accommodate future implementation of CustomField groups
• Introduce CustomFields to enable non-technical users to define new data types

Imports

• ReviewableEvents are replaced with ImportEvents, which are unified with the new Contact model
• ImportEvent inference is overhauled to provide a pluggable interface for integrations that may have better insight on how to match contacts than the default implementation
• Overhaul the API to meet new standards for usage, validation, pagination.

Permissions and roles

• Separate the concept of a "person" from the concept of a "portal user" with two models: Contact and User
• Introduce the concept of a SecurityPrincipal (onto which roles and permissions are applied) to allow for future automation that isn't necessarily associated with a user
• Fine-grained permissions and role-based authentication with those permissions

Misc

• Introduce storing settings in the database, to allow for feature flags and user-editable settings without server access
• Consistent pagination support in lists (inspired by Stripe pagination)
• Prefixed unique IDs (e.g. Contact ids are ct_xxxxxxxxxxxxxxxxx) for new datatypes to improve debuggability and distinguish identifiers (inspired by Stripe and GitHub IDs)
• Request and response validation with Pydantic models

See svdsa/home#42 for more details.