Imported and configured bi-app sass module for rtl support.

TNL-813
Replaced direction related css properties with mixins from bi-app-sass.

TNL-813
Added single-box-shadow sass mixin with rtl support.

TNL-813
Flip fontawesome's icon-caret-right 180deg in rtl.

TNL-813
Upgraded sass framework neat to v1.7.0.

TNL-813
Added bidi versions of neat mixins: span-columns and omega.

TNL-813
Sass fixes for rtl.

TNL-813
Merge pull request #639 from edx/usman/tnl813-rtl

Sass changes for RTL.
Update i18n strings.
Pulled translations from Transifex.