Add a favicon

[shepmaster]

Just to stop the silly 404s if nothing else.

[yerke]

Since the playground is now served on https://play.rust-lang.org/, I think it's not unreasonable to have the official Rust icon. As a comparison (although probably not perfect one), https://blog.rust-lang.org/ and https://thanks.rust-lang.org/ have the same icon as https://www.rust-lang.org/.

According to https://foundation.rust-lang.org/policies/logo-policy-and-media-guide/, logo can be used for non-commercial uses without additional explicit approval.

What do you think @shepmaster? If you do prefer the explicit approval, I can reach out to the foundation lawyers on Zulip/email to get that approval.

As a reminder, there is also a PR to add a dedicated favicon shaped as "P" in https://github.com/integer32llc/rust-playground/pull/704 by @imjasonmiller.

[Rudxain]

Is help wanted label still necessary? #704 seems done