Bug 1762845: HTML export still contains the 'details' / 'add comment' links for blocks on the pages

- add in render_instance_export func for comment block
- remove non-export things in render_instance_export
- add check for export status when setting canedit var

behatnotneeded

Change-Id: I3f9ae3d82c0a69277e2e6e79404f73ad9d35e3ba
Signed-off-by: Lisa Seeto <lisaseeto@catalyst.net.nz>