Dynamically select view or edit mode

The "Absence Request" screen supports view and edit modes. Currently, the inbound link must be constructed based on the desired mode (eg "action=view" or "action=update"). This is not the desired behavior.

The inbound link should always be the same (eg always "action=update" or always "action=view" – whichever is easier). Then:

• If the current user has permission "administer CiviCRM", then use edit-mode.
• If the activity status is unreviewed, then use edit-mode.
• Otherwise, use view-mode.

(Note: We may want to change the particular rules in the future - eg adding other conditions in which the activity may be edited.)