Add simple API

[BlakeRain]

This PR adds closes #15 by adding an API to Parcel. As describde in the issue, the API is authenticated via a Bearer token in the Authorization header, which the user can generate via the UI.

• Implement the PUT handler for uploads
• Implement the download handler
• Implement the POST handler to create uploads
• Add a UI for managing user API keys
• Add API test suite, probably using the poem::test module.
• Add a test for the API client
• Add E2E tests for the API keys