Create a security page

[François Marier]

We should have a security page at https://www.libravatar.org/security which includes:

- how to report security issues
- what technologies we use to improve the security of the Libravatar application

[François Marier (fmarier)]

First sketch of that page up on https://wiki.libravatar.org/security/

We'll need to:

- add a redirect from www.libravatar.org/security
- add a link somewhere in the header or footer