Migrate existing campaigns' names to the new slug-like format

Created by: mrnugget

If we decide against https://github.com/sourcegraph/sourcegraph/issues/12641 and https://github.com/sourcegraph/sourcegraph/issues/12640 we need a migration that migrates existing campaigns' names into the new sluggable format, where no whitespaces are allowed (see campaignSpec.schema.json)