Adds suffix to enum methods and changes `in_groups` to
`of_groups`

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.

- Namespace now return avatar_url and web_url
- NamespaceBasic entity was modified to include avatar_url and web_url,
information is fetched differently depending if the Namespace is a user
or a group

Includes documentation changes

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/39858

[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).

This reverts merge request !26823