Add support for call url

[chevdor]

Recently, the polkadotjs app added support for a great feature: call url.
That allows up now sharing a url that points to polakdotjs and will decode the call.

Here is an example

Clicking it opens the decoded call in PolakdotJS.
In addition, if you switch to the Submission tab, you can then modify the call and send it.

It would be nice to include those url in the proposals for instance as the decoding in PolkadotJS is better than the json we can see in Polkassembly.