Create user-flow for user management

[Narigo]

Description
A user needs to login to register or update a shop. Right now, the user management is implemented in the backend, but the frontend is still missing.

Links/References
Related to #7

Tasks

• Add a login frontend when user wants to register / update a shop
• Save login token in frontend
• Send login token with each request to the backend

Additional Context
There is a pull request for the backend already