Enable per-student background tasks.

* Make per-student background task buttons appear on the instructor dash, by using the correct name of the MITX_FEATURE (i.e. ENABLE_INSTRUCTOR_BACKGROUND_TASKS).

* Remove redundant transaction.autocommit annotation in api_helper.py, since that is handled in save_now().

* Add docstrings.

* Add two tests that will work when Mako templates are reliably initialized for tests.