Connecting the iOS app to local webapp

This MR contains several changes in the code replacing hardcoded gitter.im references with using URL from config. There is a change to Transport security settings, allowing HTTP connections to localhost:5000. Lastly, the updated README explains the necessary steps for connecting to the local webapp.

Thank you @MadLittleMods for unblocking me with the transport security settings, saving me hours if not days of struggle. ❣

Dev notes

We were having problems with the http://localhost:5000 OAuth page just being blank and not loading. After a bit of sitting on the blank, page, we would see the following error in the Xcode output. No http URLs were loading in the WebView 💢. We fixed up this issue by adding in some NSAppTransportSecurity

2020-01-10 19:43:04.826814-0600 Gitter[31420:18433252] Could not signal service com.apple.WebKit.Networking: 113: Could not find specified service