Replace core spacing classes with Bootstrap 4 Stable classes

In Bootstrap 4 Alpha

• m-t-* and other spacing utilities should be replaced with mt-*.

The units that were used for margins have changed too

BS4 Alpha:

m-t-1 {

margin-top: 16px;

}

BS4 Stable

mt-1 {

margin-top: 0.25rem; // 4px on a font size 16px

}

so this translates mostly to
m-t-1 is now mt-3 
m-t-2 is now mt-4
m-t-3 is now mt-5

There are appoximatly 316 classes to be replaced:

(git grep E '(m|p)(a|l|r|t|b)-' | wc -l)