Move locale selector to the settings, in the Customization tab;

This tab is the themes tab recycled to be more general. This commit also
enhances the theme texts, and adds labels to make it easier to click
selects.
4 jobs for move-locale-selector in 6 minutes and 36 seconds (queued for 1 second)
Status Name Job ID Coverage
  Lint
passed lint #348045

00:01:42

 
  Build
passed build #348046

00:01:46

 
  Test
passed lint_history #348048

00:01:46

passed test #348047

00:03:07