Preserve pre-existing css classes on edit tab link

Follows-up Ib2e6237483338c8.

Note that the potentially dangling space is cleaned up by the
Html class in MediaWiki which recognises this as a space-separated
attribute. It also supports an actual array, if MediaWiki core
were to expose 'class' as an array, we could just cleanly push an
extra into it (as well as easily removing items). But this hook
maintains this array item as a string for backwards compatibility
with hook listeners that expect a string.

Bug: 64646
Change-Id: Ib74ecf93fc4fc4bbb1af43efdf325edc7fb2d85f