Bug 1026184: Port /security to bedrock.

This is nearing completion. I have a question in the bug about what
other pages need porting. It's clear (to me) that some don't.


1. `./manage.py syncdb --migrate`
2. `./manage.py update_security_advisories`

Things should work normally after that.

@jgmize this is decently complete. Some things need more tests, but I'd appreciate a
quick look to at least make sure I'm on a path that will land eventually.

@sgarrity The pages aren't ugly, but could really use your touch, especially the big lists
of advisories.
See my comment on https://github.com/mozilla/bedrock/pull/2231/files#r16516181 -- you won't need this if you make the suggested change, but if for some reason that doesn't work and you actually need partially parsed data from `parse_md_file` then this should probably be in a `try` block.