feat: add user accounts

[diogogmatos]

Add user accounts to allow saving user preferences (selected filters, selected shifts, selected theme, selected configuration of a custom theme) in a database instead of relying on local storage.

Benefits:

• Same preferences across devices.
• Reduce bugs and complexity of managing local storage information.
• Local storage information can be messed with by the user, database storage can't.

Cons:

• Anonymous users would need to sign in for their changes to be persistent.
  • Possible solutions for this con:
    • Store information from anonymous users temporarily (for maintainability purposes) on the database, identifying them with a random token.