Save 'issue closed' events into the DB

[Cleop]

As a gh-backup user I want to be able to see the status of an issue as I would on github So that I navigate easily to the content I want

We are currently not tracking the event of when an issue is closed. In order for our app to reflect github's information we should include this.

• [x] Receive webhook from gh when an issue is closed
• [x] Create issue status table: see https://github.com/dwyl/github-backup/issues/70#issuecomment-373352415
• [x] Save this data to the issue status table
• [x] Display the event on the issue page at the top

The structure for doing this issue has already begun in controllers/event_type.ex but has been commented out as it is incomplete. Look for the reference to this issue in that file.

[Cleop]

Schema

As there will be numerous event types (e.g. #71), we're going to create an 'issue status' table.

event: enum  opened/closed/reopened 
user_id: reference to the user table (coming soon)
timestamp of the event
issue_id: integer issue_id from the issue table

For the time being the information will be displayed at the top of the page to signify that an issue has been closed, not in situ in the chronology of the page.

[Cleop]

Still to do:

• [ ] refine UI
• [ ] Finish test