Auto merge of #1834 - pietroalbini:security-footer, r=sgrif
Auto merge of #1834 - pietroalbini:security-footer, r=sgrif

Tweak links in the footer

This PR tweaks two things in the footer:

* A link to the [security policy](https://www.rust-lang.org/policies/security) was added: we had a person asking where to report a security issue on Discord, so hopefully this extra link should point people to the right disclosure guide.
* Removed a redirect when someone presses "Install": the old page it linked to ([`/install`](https://crates.io/install)) instantly redirected the viewer to [the cargo guide](https://doc.rust-lang.org/cargo/getting-started/installation.html), so I changed the link to point directly to it.

r? @sgrif

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io