#165273492 Add user create report of article functionality

[KabohaJeanMark]

What does this PR do?
It enables authenticated users to report articles which

Description of tasks to be completed

• An authenticated user can report articles
• An article cannot be reported more than once
• Only the admin has the permission to view the reported articles

Endpoints
POST api/article/<article_id>/report Report a specific article by its id. Authenticated user access
GET api/article/reports/ Get all articles that have been reported. Only admin access

How to test this PR

Expected request body for a post

{
	"report": {
		"reason": "Plagiarism and refusal to admit sources of intellectual property"
	}
}

Expected body for a response

{
    "report": {
        "id": 2,
        "reason": "Plagiarism and refusal to admit sources of intellectual property",
        "date_reported": "2019-05-06T15:42:37.157955Z",
        "article": 2,
        "reporter": "marcus"
    },
    "message": "Report successfully created."
}

If a person tries to report the same article twice, the expected response body is

{
    "error": "You already reported this article"
}

To view all reports of articles
GET api/articles/reports/

#165273492

[mwinel]

As per the implementation requirements on the PT board, this LGTM. It is ready for a merge.