[sharing] allow admin to set default expire date

Allow admin to set default expire date. Default expire date will be defined as "days after the item was shared".

 Options:
- set default expire date, will be used if no user defined expire date exists
- set default expire date and enforce it, use defined date if it is smaller than the default date, otherwise use the default expire date

cc @jancborchardt can you have a look at the additional admin settings. 

At the moment I don't show the default expire date in the share drop-down. The problem, the share dialog can only set one expire date for all shares of one file while the default expire date is always n days after a specific share was created. Maybe you have a idea how to visualize this settings for the user?