CON-619 Removing opacity mixins for browsers not supported anymore

In addition to autoprefixer remarks, removed khtml prefix
for Safari 1.*
CON-619 Removing unused opacity mixin
CON-619 SCSS best practices

Opacity attribute placement correction in modified files
CON-619 Code review changes
CON-619 @hakubo `s Code review changes
Merge pull request #2837 from Wikia/CON-619

CON-619 Removing opacity mixins for browsers not actively supported anymore