Fix #521 Modifying a talk using the details view hides the save button

- Enable Save Button only when an input field is modified by calling function
  enable_save
- Disable Save Button on init, and when input fields are autofilled or
  cleared
- Allow Save Button to be used for updating existing talks and adding new ones
- Change Save Button text to 'Save Talk'
- Edit docstring for function unsaved_details_exist