Add support for dynamic environments

Implements proposal described in https://gitlab.com/gitlab-org/gitlab-ce/issues/21971.

Specifically:
- it adds a `.gitlab-ci.yml` configuration,
- it allows environment name to have slashes,
- it allows environment names to use CI predefined variables,
- it allows to specify URL from `.gitlab-ci.yml`,
- it allows the URL to use CI predefined variables,
- it introduces `environment_type` to allow to easily group environments in the future

See merge request !6323

Ensure validation messages are shown within the milestone form

Fixes a bug where upon entering data to create an invalid milestone via the Web UI or the API would raise an exception rather than render the validation messages

We'd rather render the validation messages to inform the user than raise an exception and result in a 422 error being displayed

Closes #22033

See merge request !6345

Reset pushes_since_gc counter before specs run to ensure starting point is 0

See merge request !6406

Render invalid template for open merge requests without source project

If source project was removed and merge request for some reason wasn't closed is rendered `invalid` template. Fix !6177

Fix #22248 

See merge request !6383

Add missing spec for ProtectedBranches::CreateService

While working on !6399, I noticed there was no spec for this class so I added one.

See merge request !6405

* Remove call to Milestone#save! instead just Milestone#save
* Add safety specs to stop a regression

Fixes #22033

Refactor boards_spec.rb to avoid code duplication

Originally created to address #21197, but !6224 solved the intermittent spec failures.

See merge request !6197

Squashed commits:
[f9a9315] Use : to test invalid environment name

Environments that can have a URL with predefined CI variables.

Return created status

## What does this MR do?
Properly handles status created which results in 500 when viewing Merge Request pipelines.

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

## Why was this MR needed?

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- 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 you do - 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?
Closes #22286

See merge request !6404

Document how to download the latest build artifacts

## What does this MR do?

Document how to download the latest build artifacts.

Closes gitlab-org/gitlab-ce#22273

See merge request !6401

[ci skip]

Update GitHub importer documentation  to reflect current importer state

## What does this MR do?

Update the Github importer documentation to reflect the current importer state.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/904

## Does this MR meet the acceptance criteria?

- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)


See merge request !5987