Allow button text to be configurable

[swashbuck]

Subject of the issue

The button text should be configurable. Currently, it pulls from Adapt.course.get('_buttons')._submit. I would suggest something like:

"_submitAll": {
  "_isEnabled": true,
  "_insertAfterBlock": "",
  "_button": {
    "buttonText": "Save & Continue",
    "ariaLabel": "Save & Continue"
  }
}