* remove trigger from end_date
* adds new function `get_end_date`:
- it tries to calculate the appropriate end date for a given frequency
- returns an empty string if frequency is 'biweekly'
- adds test cases
* changes logic in `set_start_end_dates`:
- if start_date is empty in form, call process_payroll.get_start_end_dates
- else, call process_payroll.get_end_date
* `get_end_date` should return a dict
* changed "biweekly" to "bimonthly"
* change the behaviour of process payroll start and end date:
- when payroll frequency is changed, change start/end date as usual
- if start date is manually changed, use the frequency to calculate the end date
* clean up
* further cleanup
* in `get_end_date`, if `frequency` isn"t given, "monthly"
* remove end_date from cscript and introduce `set_end_date`
* fix tests
* removed whitespaces
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