Rename btn-new, btn-create and btn-save to btn-success

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

When using ruby interpolation in externalized strings, they can't be
detected. Which means they will never be presented to be translated.

To mix variables into translations we need to use `sprintf`
instead.

Instead of:

    _("Hello #{subject}")

Use:

    _("Hello %{subject}) % { subject: 'world' }

# Conflicts:
#   app/views/projects/mirrors/_instructions.html.haml