Migrate team invite signup to React - Part I (Backend)

[paolodamico]

Changes

This PR lays the ground work on the backend to migrate the team signups (when a team member signs up to an existing organization) endpoint to React & DRF. Among other things, we intend to simplify these flows & reduce tech debt, plus improve the overall UX (and therefore increase internal referrals). Every new functionality includes applicable functional and unit tests.

A follow-up PR will implement the frontend stuff for this PR, plus remove all the legacy code.

Details:

• Migrates all relevant endpoints to accept invites to DRF which now validates everything via an API request.
• Allows to pre-validate invites so that the user doesn't have to re-enter information we already have.
• Makes use of the recently introduced standardized event usage instrumentation.
  • Fixes an instrumentation bug on how some extra attributes were reported on signup-related events.
• Adds back-end support to process invite signups with social auth.
• Renames all references to company_name to the less confusing organization_name.
• Introduces .active_invites property on organization objects to filter only active invites.

Checklist

• All querysets/queries filter by Organization, by Team, and by User
• Django backend tests
• Jest frontend tests. Not applicable.
• Cypress end-to-end tests. Not applicable.

[paolodamico]

Cloud tests fixed on https://github.com/PostHog/posthog-production/pull/74

[Twixes]

Things seem proper