CBG-1893: DELETE attachment endpoint

[gregns1]

CBG-1893

• Added a new DELETE endpoint to give the functionality to delete an attachment from a document without going through the PUT attachment endpoint
• Added some more functionality to the existing test for testing document attachments
• Updated the api documentation to reflect the changes being made to the endpoints

Pre-review checklist

• [x] Removed debug logging (fmt.Print, log.Print, ...)
• [x] Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• [x] Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• [ ] xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/886/