pelican_plugins/bootcamp_meta.py: Parse EndDate metadata

Give it the same processing used for 'date' metadata.
theme/templates/bootcamps.ics: Add iCalendar template

There are a number of unfinished issues:

* I'm waiting to see how my keep_trailing_newline patch against Jinja
  plays before dealing with the lack of a newline at the end of this
  template [1]
* I'm not sure about how to enable CR-LF newlines for a single
  template in Pelican, because Jinja normalized newlines internally.
  Maybe using CR-LF everywhere is the way to go?

[1]: https://github.com/mitsuhiko/jinja2/pull/170