Get/set multiple settings entries at once

Add two functions to RccSettingsService that let us get/set multiple settings at once.

Motivation:

REPOSE participant info comes into the app as a ReposeData object (see ReposeDataService):

export type ReposeData = {
	'repose' : {
		respondent_id	: string
		passphrase	: string
	}
}

Handling this is a bit tedious, because we have to set both settings values individually. When getting, we have to make two requests to RccSettingsService and reconstruct the object from there. It would be easier if we could get several settings entries at once.