Add config field to set max size of uploaded files by admins

Is your feature request related to a problem? Please describe.
As an admin, I sometimes want to upload heavy files but I don't want to allow that to the users. 

Describe the solution you'd like
add a field to the "Maximum file sizes" in the system organization edit form so that we will have two fields to indicate the size : one for the admins and one for the users.  

Additional context

Does this issue could impact on users private data?
No

Funded by

Could be funded by Open Source Politics.