Redirect to local openHAB server

[mueller-ma]

There are several use cases to be able to create "deep links" to something in openHAB, e.g. a subpage of a Sitemap or a tab in Main UI. These use cases include:

• Specify a target for the press on a push notification (https://github.com/openhab/openhab-android/issues/3193)
• Customization for the native device control on Android (https://github.com/openhab/openhab-android/issues/3140)
• Open addon installation page from openhab.org/docs

While you could use the links that Main UI and Basic UI in the browser use, there are two reasons not to do so:

1. The host name may change based on the location of the client (e.g. local or via myopenHAB)
2. AFAIK Android apps cannot register themselves to handle URLs during runtime, but only during compile time.

@ghys came up with https://my.home-assistant.io/. It's a webpage where you configure the URL for your local home-assistant server and get redirected to it, e.g. "Add a zwave device": https://my.home-assistant.io/redirect/add_zwave_device/

We could do something similar, either on the website directly or in a new project. It should be under a separate domain like link.openhab.org, so the Android app (and others) can register themselves to handle links to this website. Android apps can also register for a path on a website, e.g. openhab.org/link