Allow users to select preferred license for their contributions

Needs some UI love on both selection & intimation, but works otherwise

People can pick from CC BY-SA, CC BY, or CC0. The commons templates
are set using {[self}}, which I think is good enough.

GitHub: https://github.com/wikimedia/apps-android-commons/pull/14
Change-Id: Iffca6f75c63de6fbe61c4dc41b93dd5f692065a5