Rename files to align with Bootstrap viewports

- Replaced WET viewport variables with references to the Bootstrap versions. File names were renamed to match
- Moved the Calendar theme file as it wasn't referenced before
- Renamed core SCSS files to `_base.scss` except for the polyfills that are supposed to output
- Renamed print view to `_print.scss` and `_print-ie8.scss`
- Rename IE8 fallbacks to `_ie8.scss`
- Combined the Details/Summary bases that where in src/base with the polyfill's SCSS

Not sure if this will impact the themes.