Corrected weekly_report.tcl to respect page contract: "- Have the permission to add absences AND have at least some absences logged"

  names, added two new parameters

  to avoid error messages

  to include monthly, weekly _or_both_.

  membership in projects.

  a custom timesheet implementation

- Added a link to the headers of the weekly view to go to
  daily views

  - Now calculating the duration days of the absence taking into account
    user for whom the absence is created
  - Calculate duration calculation: All arguments are now checked.

  - Vacation Balance Portlet:
    Now showing a warning message and a link in any vacation balance
    portlet when it's necessary to update users.
  - Vacation Balance Report:
    Only offers to update the balance for those users with an outdated
    balance.
  - Added lengthy help text
  - Improved clarity of header0

  yearly balance

- Created new absence type "weekend" with own color

- Added weekend color