Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/58023

- docs for adding Saturday for first day of the week
- add related settings for Saturday as first day of the week
- firstDayOfWeek: Use enumeration, replace day's numbers with
  corresponding names
- make some variables lowercase (follow camelCase)
- add CHANGELOG entry

Author:    haghighi_ahmad <haghighi.ahmad@gmail.com>