Document qitem endpoint in openapi.yaml for editing quarantine mails

[jonprocter]

Added endpoint to edit quarantine items with actions to release or learn as ham. Functionality already existed but was undocumented.

Contribution Guidelines

• I've read the contribution guidelines and wholeheartedly agree them

What does this PR include?

Short Description

Just documented the already existing edit/qitem API endpoint. It supports release and release + learn ham, but wasn't documented in the yaml.

Affected Containers

N/A

Did you run tests?

Tested by discovering the API calls worked, and then documented.

What did you tested?

These API calls:

curl -X POST https://mail.mailcow.tld/api/v1/edit/qitem \
  -H "X-API-Key: api-key-string" \
  -H "Content-Type: application/json" \
  -d '{
    "items": [649, 650],
    "attr": {"action": "release"}
  }'

curl -X POST https://mail.mailcow.tld/api/v1/edit/qitem \
  -H "X-API-Key: api-key-string" \
  -H "Content-Type: application/json" \
  -d '{
  	"items": [700, 701], 
  	"attr": {"action": "learnham"}
  }'

What were the final results? (Awaited, got)

They worked, so added to openapi.yaml.