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)
## 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
f46d5f5813