Add favicon to essential files

[jyn514]

This can be tested with cargo run build add-essential-files && cargo build chrono 0.4.18.

Closes #1070.

r? @Nemo157

[Nemo157]

So when I was working on the static files I realised that the per-version favicon.ico is redirected to our favicon.ico, should we do the same and add a static favicon.svg that we redirect to?

[jyn514]

From our conversation on discord:

• The complexity of special-casing the favicon doesn't seem worth the benefit (~100 copies of the favicon is trivial compared to the rest of our costs)
• Storing a single static favicon would miss changes in new versions and we'd have to update manually.

It might be nice long term to look into de-duplicating files in general, which I imagine would have enormous storage cost reductions, but sounds hard to do.