server: Clean up flow and more

[infinisil]

This cleans up many parts of the demo server implementation:

• Moves all Haskell files to server/src
• Moves all webserver files to server/www
• Have a dedicated page for logged-in users
• Allow users to log out again
• Don't require a display name when registering
• Have entr run yarn build again when files are updated
• Don't create user entries in the database until the registering is complete
• Use Scotty.liftAndCatchIO instead of liftIO so that Exceptions are thrown via HTTP

[infinisil]

Closes #34