core: i18n splash screen.

- Added SplashServlet to i18n the splash page.
- Added LocaleFilter to automatically detect locale based on
browser preferences, or parameters. Order of importance:
  1. parameter.
  2. Cookie.
  3. Browser preferences.
  4. Default (en-US).
- Added a LocaleUtils class to capture common methods.
- Added triangle_down_gray image.
- Added unit tests for all used classes.

Change-Id: I37156061cbdd7d2df3290c88dee933c41e0087c6
Bug-Url: https://bugzilla.redhat.com/838456
Signed-off-by: Alexander Wels <awels@redhat.com>