Make CI to use the permission of the user who is trigger the build

This is continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5735, but with removed all LFS code that is added by: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6043.

This also incorporates most of LFS code added in !6043 to simplify further merge.

See merge request !6409

Fix import/export database errors

Fixes protected branches errors when importing a project including them

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/21295

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/21799

See merge request !6099

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.

Better import of labels, milestones and protected branches. Updated relevant specs.
Loose pipeline validation on importing, so it does not fail when there are missing fields, which are not validated at DB level. Also, updated spec with relevant test.