user authentication

[facundoolano]

• Adds a user table to the db and a user foreign key to the feeds table
• Adds flask-login and views for authentication
• Requires login for all routes
• Update all db queries to filter feeds by the authenticated user's id
• Update cli commands to take a user (e.g. to export/import feeds from a file)
• Adds cli and make helpers for user management
• Insert and log in a default user to effectively disable auth by default (so local/single user setups don't need to be bothered by it)