Comments API + attachments updates

                      
|Q             |A    |
                      
|Bug Fix?:     |n    |
|New Feature?: |y    |
|BC Breaks?:   |n    |
|Deprecations?:|n    |
|Tests Pass?:  |y    |
|Fixed Tickets:|     |
|License:      |GLP-3|
|Doc PR:       |     |
                      

Provide REST API for Comments resource + fix some attchments stuff