Implement client auth flow

[controversial]

This PR builds upon #88 and #91 to store login information in the global application store. Closes #59.

Note: #88, #91, and #92 should be merged before this is merged.

• [x] Updates the SignIn page to post user credentials to the /api/v1/token endpoint
• [x] Redirects users off of sign in page after successful authentication
• [x] Adds auth.accessToken and auth.refreshToken to the global application store's state
• [x] Adds a 'login' action to the global application store which mutates those pieces of state
• [x] Adds a 'logout' action to the global application store which resets those pieces of state
• [x] Mirrors stored auth credentials in localStorage to persist across application reloads
• [x] Updates API functions to automatically send token credentials with requests
• [x] Updates API functions to automatically refresh expired API tokens before sending
• [x] Renders the SignIn page in place of any other pages whenever auth credentials are either missing or invalid and unrefreshable