Add permalinks for icons

[syncopsta]

Add a way to generate permalinks for a generated icon.

[sebinside]

Ideas from the stream chat:

• liketechnik: ?id=X&backgroundColor=ffffff&foregroundColor=ffffff
• MelanX: download=1
• daniel_212: URL and icon config should always stay consistent

[SimonFischer04]

As there was some more discussion later on about encoding the whole store in the url:

Maybe look at what mermaid.live is doing. They encode the whole diagram in the url, so I think we shouldn't run into any URL-length limitation with the few properties required here.