[FEATURE] Simple URL-based deployment

We should support the use of the CARP Studies app in simple stand-alone studies running without using the "big" CARP Web Services (CAWS) setup.

This can be supported by putting the JSON study protocol on a website and downloading it to the app, and deploying it locally using the `SmartphoneDeploymentService` - just like when the app runs in `local` deployment mode.

The protocol specifies where to "upload" data, incl. storing locally on the phone (using the SQLite DB). It can also use an alternative endpoint (other than CAWS), like Firebase.

## User Stories

* As a participant, I want to click a link or scan a QR code so that I go directly to a study.
* As a researcher, I want to put my JSON protocol on a web URL for participants to access directly.
* As a researcher, I want to specify my own backend (in the protocol), so I get my data where I want (not CAWS), incl. a local SQLite backend.