feat(rest-api): Extend task docu

GET /task/{id}/comment
    returns all comments for task

GET /task/{id}/comment/{commentId}
        returns particular comment for task

POST /task/{id}/comment/create
     add comment for task and return comment object with link for
     the request

related to #CAM-2134, #CAM-2135, #CAM-2148
chore(how-to):update cockpit plugin mvn dependency

refer to #CAM-2164
fix(enterprise):fix previous download link

fix download link for standalone webapp download version 7.1.0-Final
chore(how-to): add tomcat hint to cockpit tutorial

add context path hint for apache tomcat to cockpit plugin tutorial
feat(web modeler): add new release of web modeler

This adds the new release of camunda web modeler to enterprise section

Related to: #CAM-1982
fix(web modeler): change year of new release
impr(rest-api): improve task comment resource documentation

related to #CAM-2134, #CAM-2135 and #CAM-2148
impr(bpmn20): add doc of camunda properties added by camunda modeler

related to #CAM-2110
fix(rest): missing status codes
chore(ext): add grails community extension
chore(how-to): extend hint for tomcat cockpit plugin deployment

related to #CAM-2145
feat(rest-api): Extend task dou for attachment

GET /task/{id}/attachment
    returns all attachments for a task

GET /task/{id}/attachment/{attachmentId}
    returns particular attachment for a task

GET /task/{id}/attachment/{attachmentId}/data
    returns content for an attachment of a task

DELETE /task/{id}/attachment/{attachmentId}
    delete a particular attachment of a task

POST /task/{id}/attachment
    add attachment to a task and return attachment object
    with link for the request

related to #CAM-2136, #CAM-2137, #CAM-2138, #CAM-2139, #CAM-2140