workshops/templatetags/workshops_person.py: Add a num_taught filter

My previous Count() suggestion implemented in 03ecafeb (Annotating
instructors with number of times taught, 2014-12-18) was removing
folks who had never taught.  This commit replaces that approach with a
custom filter [1], which makes it easy to also display other counts
(e.g. if you want to see how often someone's helped out).  It also
lets you isolate the calculations in the UI, since the view logic
doesn't care how many times you've taught.

[1]: https://docs.djangoproject.com/en/1.7/howto/custom-template-tags/#writing-custom-template-filters