Validate value for expiration date offset (settings) in frontend

Background:
As defined in UIU-1908, a user can save an offset value in days in the patron group settings table.
This value has to be an integer >=1.

Scenario:
Given: Into the input field "Expiration date offset (days)", a user enters a value other than a positive integer, e.g. a decimal or a letter.
Then:

• The input field should be marked red and
• a red message should display underneath the field, reading "Offset must be a positive integer value"
• The "save" button should be disabled until the value is either correct or the field is blank.