Created by: teoucsb82
Per https://github.com/twbs/bootstrap/issues/19443
Cleans up lingering doc/scss references to .center-block (post https://github.com/twbs/bootstrap/pull/19102)
Per https://github.com/twbs/bootstrap/issues/19343
Added new .display-<display-type> classes
Request was to combine them in one PR, but they're in two commits if either feature is wanted without the other.