Add a slug to environments

## What does this MR do?

Adds a `slug` field to the `environments` table, populating existing rows and ensuring that new rows will get an entry.

Cleaning examples:

* `review/foo`  => `review-foo-5gghdf`
* `review-foo` => `review-foo`
* `1-foo` => `env-1-foo-e2hx12`
* `production` => `production`
* `Production` => `production-f8ddlz`

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

This migration requires downtime. I don't see a way to avoid it.

## Why was this MR needed?

External services often have more restrictive rules on naming than those enforced for `environments.name`. In particular, forward slashes and names longer than 24 characters causes problems on OpenShift. `slug` is designed to be an acceptable alternative to `name` in these situations. Since forward slashes are a documented part of environment names, to set environment types, we need an envionmnent slug, not just a slug for the branch name.

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [X] API support added
- Tests
  - [X] Added for this feature/bug
  - [x] All builds are passing
- [X] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [X] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [X] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [X] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Part of #22864

See merge request !7983