Use reasonable default date for medication fulfillments

### Story ###
- https://www.pivotaltracker.com/story/show/72793588

### Notes ###
- fixed issue with non-UTC moment usage in <code>serialize()</code> view method
- added default date method for populating fulfillment <code>dispense_date</code> and <code>dispense_time</code> based on the following logic
  - if the patient data criteria has a <code>start_date</code>
    - if the patient data criteria has at least one fulfillment
      - if the patient data criteria has both <code>dose_value</code> and <code>frequency_value</code>
        - then compute the number of days given the <code>dose_value</code>, <code>frequency_value</code>, <code>quantity_dispensed_value</code>, and fulfillment <code>dispense_datetime</code> and use that as the offset
      - else set the offset to 15 minutes after the latest fulfillment <code>dispense_date</code>
    - else set the offset to 15 minutes after the <code>start_date</code>