CAPX-28: Mapper in use block

# Ready for review
dependant on: #24 

#### Adds a mapper in use block to the top of mapper pages.

To test:
* Create new mapper
* Edit that new mapper and see no block at top of page
* Create or use existing importer and select the newly created mapper. Do this two or three times.
* Edit that new mapper again and see block at the top of page

A small change to the importer in use block snuck in here as well to prevent it from showing content if nothing to show.