Add an URL field to Enviroments

## What does this MR do?

Adds a field to the `enviroments` table to expose later in other features. Now I see the task list below, I noticed I forgot some minor things, but Ill adress those after the first review. 

## Are there points in the code the reviewer needs to double check?

The field is a string on the database, thus limited to 255 chars, which seems more than enough.

## What are the relevant issue numbers?

Closes #19527

## Screenshots (if relevant)

![Screen_Shot_2016-07-25_at_15.22.16](/uploads/5c3f39466f8e2114195270006aed20c0/Screen_Shot_2016-07-25_at_15.22.16.png)![Screen_Shot_2016-07-25_at_15.22.53](/uploads/408cf86610137dd4a861d15dcd677a2c/Screen_Shot_2016-07-25_at_15.22.53.png)![Screen_Shot_2016-07-25_at_15.23.10](/uploads/927c506931f4e0338cdbbc58678bd308/Screen_Shot_2016-07-25_at_15.23.10.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5469