Greater standardization of sizing terminology

In class names:
* large => lg
* small => sm
* mini => xs ("Extra small")

In screen size categories:
* Tiny => Extra small
rm stray space in components.html
add .btn-xs to .btn-group example
add missing ref to .btn-xs in button sizes docs
Merge pull request #9056 from twbs/large-to-lg-etc

Greater standardization of sizing terminology