Bug 1637371 - Consilidating raw theme colours

- Amend Ocean theme implementation similar to other theme so new theme
  can inherit from it

- Ensure that all colour values in scss components are variables (except
  those that are more specific)

- Remove select2.scss as it is now part of raw theme for theme-ability
   - Also remove select2.css inclusion in PHP files
   - Add scss version of select2 styles and remove existing one
     (_select2.scss)
   - Create _mahara-select.scss for style override

behatnotneeded: styling only

Change-Id: I624b55ce34dd19ad525ba250877780bfbc0251b5