whitelabel overhaul
This is a major overhaul of the whitelabeling support using gradle build flavors. It sets up two build flavors "full" and "basic". "Full" is F-Droid as we know it today. "Basic" is the most minimal possible build of F-Droid that does something. The "basic" flavor currently does not even have a view to browse/search apps, but it can be used as the basis for #48 (closed).
This also overhauls the main preference handling to get rid of technical debt, and allow the preference defaults to be easily set in a build flavor or whitelabel version.
Edited by Hans-Christoph Steiner