"Data synchronization schedule" field enhancements

Purpose:

The "Data synchronization schedule" field has some flaws:

• lets enter non-positive values for time interval
  
• shows "Runs every minutes" if the value is not defined
  
• says "minutes", "hours", "days", ... if value is 1
  
• "minutes", "hours", "days", ... are not translated
  
  

User story statement(s):

As a user,
I want "Data synchronization schedule" field to be more consistent
so that the form UX be better

Scenarios:

1. Scenario:
   • Given the form with "Data synchronization schedule" field
   • When user tries to enter anything except positive, non-zero, non-fractional value for time interval
   • Then it would not be possible (variant: validation error message appears)
2. Scenario:
   • Given the value for time interval is not defined
   • When user views the configuration
   • Then "Data synchronization schedule: Not set" is shown
3.  Scenario:
   • Given the value for time interval is defined
   • When user views the configuration
   • Then the plural form of time unit ("minutes", "hours", "days", ...) fits to the time interval value
4. Scenario:
   • Given the language of the UI is not English
   • When user views or edits the configuration
   • Then the time unit names ("minutes", "hours", "days", ...) should be translated to the current UI language