Add support for user defined page controller templates

FYI: I know that normally we would use new migrations, but I want to make an exception in this case and update our existing migration. I'd like to keep all our basic table changes together, and this makes the up-coming MOD conversion migrators a little easier to manage, and also because nobody should be using this ext yet - it simply is not in a usable state yet - so I'm not concerned yet about migration files being modified vs. adding new migrations at the moment.
Comment error? These are TWIG comments. What's the error?