Add favicons

[imjasonmiller]

From #321 (comment):

My opinion is that it should be distinct but related to the favicon on https://rust-lang.org.

A possible alternative to the Rust language favicon, but using a "P" for playground instead:

The colors used are those of:

• Ferris: #f74c00
• Rust Playground background #e5e5e5

The vector graphic, while newly created, is based on the original Rust logo, licensed as CC-BY.

It can be downloaded as <svg> here: favicon.zip

Generated via https://realfavicongenerator.net, this pull request also includes favicons for:

• Desktop
  
• iOS
  
• Android
  
• Windows
  
• Safari
  

[Canop]

This is admirable.

Using a different color to avoid confusion with the many other sites having a favicon based on the rust cog is an excellent idea.

I can only approve this change which would make me gain a few dozens minutes every hour when looking for the playground tab among a few dozens millions other tabs.

[Stargateur]

Nothing prevent us to change it later, so I think it's a very good start.

[sistemd]

Nice icon

[FedericoStra]

Any update on this?

[knanao]

I'm looking forward to being merged these changes. 🦀

[Rodrigodd]

Also looking forward to this! And if an opinion is welcome, I think it would be better if the icon has the same color as the Run button (#aa4422), it would match the site better.