In accordance with https://www.notion.so/coderhq/v2-Frontend-State-Management-APPROVED-e5ccb3c6357649f9a9dc1a1b2a1b7a99, we will use a machine to encapsulate API calls about the current user and hold user state.

• replace UserContext with userMachine
• refactor all checks for authentication
• refactor SignInPage to use userMachine

Requirements:
Given a current user created in the database,

• user can log in
• user can log out
• logged in user can view pages that require authentication
• logged in user on /login gets redirected to /projects
• logged out user cannot view pages that require authentication, and is redirected to /login. Upon logging in, the user is returned to the page they were trying to access.
• logged out user can view /login and 404 page

Implementation goals:

• install XState and related packages
• set up dev tools - half done, see Improve XState dev tool setup #462
• set up typegen - half done, see Debug and automate XState typegen #456
• replace user Context with user xservice
• use xservice only in glue components

Testing goals:

• Storybook NavbarView
• Storybook SignInForm
• TBD mix of unit, integration, e2e tests to cover auth behavior