AA-184: Fix extension dashboard for self-paced courses

- Have it load dates from edx-when, not just write to it. This
  fixes self-paced courses where edx-when is only place dates are
  kept.
- Have it read original date for a homework from edx-when when
  resetting a date. This fixes the message it gives the instructor
  about whether it was successfully reset.
- Have it recursively set a date, rather than assuming that dates
  are only ever set on the subsection layer. This fixes setting
  dates on self-paced courses (where dates are set all the way
  down) and just in case somebody somewhere edits the course xml
  to have a date where it's not expected.