Add env_var and file as supported types for CI variables. Variables of
type file expose to users existing gitlab-runner behaviour - save
variable value into a temp file and set the path to this file in an ENV
var named after the variable key.

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/46806.

This is step one of resolving
https://gitlab.com/gitlab-org/gitlab-ce/issues/56838.

Here is what changed:
- Revert the security fix from bdee9e8412d.
- Do not leak repository information (tag name, commit) to guests in API
responses.
- Do not include links to source code in API responses for users that do
not have download_code access.
- Show Releases in sidebar for guests.
- Do not display links to source code under Assets for users that do not
have download_code access.

GET ':id/releases/:tag_name' still do not allow guests to access
releases. This is to prevent guessing tag existence.

Only prometheus can be uninstalled atm, the rest will be dealt with
later.

Presumption is that new application types will have uninstallation
implmemented at the same time.

Model.new.attributes now also returns encrypted attributes.

Update merge request widget to accommodate post-merge pipelines

See merge request gitlab-org/gitlab-ce!25983

This commit adds two new warning messages to the MR widget that handle
cases involving merge request pipelines.

[Backport] Support multiple assignees for merge requests

See merge request gitlab-org/gitlab-ce!27089

This is resolving https://gitlab.com/gitlab-org/gitlab-ce/issues/30157.

Implement new API endpoint `/projects/:id/environments/:environment_id`
to expose single environment. Include information for environment's last
deployment if there is one.

Backports https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10161
(code out of ee/ folder).