Add DMN configuration checks

[sergei-maertens]

If any form makes use of DMN logic actions, there should be a test in the digest email that checks the correct Camunda configuration:

• [x] Test connection/credentials to Camunda engine
• [x] Check permissions to read decision table meta-information
• [x] Add checks to configuration overview page
• [x] Check input/output variable mapping
• [ ] Add checks to digest email

[joeribekker]

Refinement: Blocked by digest email restructuring.