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.
Based on what I've now seen further down in this diff, I see that if the metadata were missing entirely then the parsing would break later anyway due to missing fields, so validating the yaml here could be considered overkill. It still makes more sense to me for this function to return the fully parsed data, but I'll leave it up to you whether you want to make that change.