workaround bug of urihandler in firefox

this line was creating the error: 
```
NS_ERROR_XPC_JS_THREW_STRING: Permission denied to add file:///Users/ematiu/devel/node/copay/null/#/uri_payment/%s as a content or protocol handler'Permission denied to add file:///Users/ematiu/devel/node/copay/null/#/uri_payment/%s as a content or protocol handler' when calling method: [nsIWebContentHandlerRegistrar::registerProtocolHandler]'
```

which prevent firefox to join wallets. This PR temporary disables uriHandler.