Implement user authentication

[schrer]

User accounts should be added and authentication should be required (at least to add/edit recipes and categories) to be able to run the cookbook online without risk.
New recipes should have an author assigned, that is a valid user. This can later be a search criteria for advanced search.

[schrer]

First inspiration for password encryption. Deeper digging needed.
https://pboyd.io/posts/how-not-to-store-passwords/