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.
Because of the ambiguity of the `---` token, I think that in addition to only using the first 2 occurrences, it's probably a good idea to validate that `yaml_lines` are actually yaml, and if it fails prepend the lines to md_lines instead:

```python
try:
    parsed_yaml = safe_load(''.join(yaml_lines))
except ScannerError:  # might be others we want to catch as well
    parsed_yaml = None
    parsed_md = md(''.join(yaml_lines + md_lines))
else:
    parsed_md = md(''.join(md_lines))
return parsed_yaml, parsed_md
```
This change would also simplify the [update_db_from_file](https://github.com/mozilla/bedrock/pull/2231/files#diff-d44cad9752ace686110b58e908e22c13R151) because it would be getting fully parsed data from this function.