Rename `.col-xs` to `.col` + some other cleanup

Created by: Starsam80

I changed lots of little stuff but the main points are

• Use breakpoint-min instead of a counter: This cleans up the code quite a bit and makes it more readable.
• Add > .col, to the .no-gutters class. Without this, > [class*="col-"] wouldn't match .col, and changing it to something like > [class*="col"] would make it match .collapse and more.
• Rename .col-xs and .col-xs-auto to .col and .col-auto
• Update some documentation to use new classes that weren't changed before
• Fixed a minor bug where even though $enable-flex was false, it would still produce parts of flex classes.