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.

@dblessing @DouweM

BTW; should be included in the current release and the previous.

Fixes #406 (closed)