* add a days_early_for_beta inherited policy option
* students in the beta_testers_{COURSE} group get to see content that many days early
* Clean up some docstrings related to time formats
* Add basic test

- also updated some out-of-date docs

- needed for cert generation (and later lots of other things)

Add discussion tag

* still needs better error checking and testing

* specify in tabs list in course policy
  - active page tracking now done in tabs.py
  - properly handle the fact that there may be multiple textbooks

* Still need:
  - wiki pages
  - (if that's delayed, special-case syllabus support)