Deprecate append/prepend utility classes

Paul Slaughterrequested to merge
217418-deprecate-append-prepend-classes into master

What does this MR do?

Deprecates append-* and prepend-* utility classes.

Why?

  • This comes up a lot in code review since there's a lot of preexisting code that still uses it.
  • We are wanting to move towards a consistent utility library provided by gitlab-ui.

Related to #217418

Edited by Paul Slaughter

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