Automated response: main_content block 'zone' parameter does not work for all content types

On review, it was not functioning for the following content types: catalogue_entry, post, group, forum. These are now fixed.

Additionally the following is noted, with no changes:
1) it is not implemented for comcode_page, where it doesn't make sense (a page has its own implicit zone).
2) For 'member', it is conceivable we would implement it, but linking to members happens all over Composr, so making the block use a specific zone would be inconsistent with member links shown in many other places - and it would require deep changes in internal APIs that are not desirable.