Set OpenResponse component to complete if student attaches a file

[hirokiterashima]

Also fixed issue where previous attachments were not displaying below the textarea.

Test the following:

• When OR component student attachment is enabled:
  • student types in response only => complete
  • student attaches only => complete
  • student types in response and attaches => complete
  • student does not type in response and does not attach => incomplete
• When OR component student attachment is disabled:
  • student does nothing => incomplete
  • student types in response => complete
  • student does not type in response => incomplete

Also test that the attachment shows up when you leave the step and come back. This was a bug that I found while working on this feature.

Resolves #46