Web Based Dapps (#3956)

* Dapps web

Conflicts:
	dapps/src/apps/fetcher.rs
	dapps/src/handlers/fetch.rs

* Rewriting fetch

* Parity-wide fetch service

* Obey the limits and support cancellation.

* Removing temporary files.

* Actually use Fetch for dapps

* Re-implementing file fetching to avoid temporary files.

* Serde to 0.8.19

* Fixing content & dapps fetch

* Serving web content and injecting scripts

* Don't wait for old content, start a new download

* Supporting timeouts and query

* Simple GUI for the browser

* Proxy tokens validation

* Recovering from invalid web-based requests

* Remember last visisted URL

* Removing unused variables

* Addressing review comments

* Setting default account in web3

* Adding WebBrowser dapp to the list

* Actually prune old entries when generating new token

js/src/api/rpc/signer/signer.js

@@ -37,6 +37,11 @@ export default class Signer {
       .execute('signer_generateAuthorizationToken');
   }
 
+  generateWebProxyAccessToken () {
+    return this._transport
+      .execute('signer_generateWebProxyAccessToken');
+  }
+
   rejectRequest (requestId) {
     return this._transport
       .execute('signer_rejectRequest', inNumber16(requestId));