Only rename the column if its not there yet
The problem can only arise when upgrading from a CE version to an
EE version after last months release as in that release the table
appearances was created. But when it was created we already took
care of this rename. Thus on an upgrade the migration will fail as
the column light_logo is not in the table, and header_logo already
is.
Thus this check solves it.
BTW; should be included in the current release and the previous.
Fixes #406 (closed)