Use ethcore_dappsPort when constructing URLs (#3139)

* Upon connect, retrieve the available api ports

* Update dapps to load from dappsPort

* Update dapps summary with dappsPort

* Allow proxy to use dappsPort

* Replace /api/ping with HEAD /

* Dynamic port for available apps

* Retrieve content images with dappsPort

* Fix /

* Transfer token dropdown image fix

* IdentityIcon loads images via contentHash

* Update apps fetch to cater for dev & prod

* DRY up 127.0.0.1:${dappsPort} with ${dappsUrl}