Bug 22249: Mana - Move comment process in a dedicated sub

Bug 22249: Mana - Move comment process in a dedicated sub

Test plan:
  - Go to a subscription's detail page,
  - click on "Report mistake" => "New comment",
  - check Koha logs file,
  - you can see the following warning:
    Can't locate object method "find" via package "Koha::Resource_comments"
  - Apply this patch,
  - same test again,
  - no wraning

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>