Add nickname editor

This PR fixes #18.

1. Add nickname EditText
2. Make "Start Scanning" button *HUGE* (because why not?)
3. Consolidate some UI strings
4. Remove "View Scan Map" button (because most users will only care about the leaderboard and the world map is not very usable in a mobile browser)

NB: The nickname editor adds some StrictMode violations, but someone can fix them later. <:)