Update frontend styling

Frontend now uses ChihuahUI and most pages (except statistics pages) are
styled. To accomplish this, some other changes had to be made:

 - Spotless frontend was added.
 - A GNU AGPLv3 license header was added to all files.
 - The old frontend has been renamed to archive.
 - The project now effectively is just the old backend.
 - Some weirdly named properties like `GITLAB_SERVER_URL` were moved to
   regular configuration properties.
 - Liquibase was added.
 - The existing authentication configuration has been replaced with
   regular LabraDoor authentication.
 - All endpoints have been changed from camelCase to kebab-case.
 - All templates have been changed from camelCase to snake_case.
 - All packages have been changed from camelCase to lowercase.
 - FancyPrint has been deprecated in favour of Logger.
 - API controllers have been deprecated, as the relevant logic there
   should be moved to service methods.
 - A simple error handler controller was added.
 - Some dependencies were updated.
 - Some non-working tests were disabled.
 - A YAML file was added in favour of properties.