API: Version information

## What does this MR do?

Adds a new endpoint to retrieve the version information.

## Why was this MR needed?

Clients can now use this information to enable/disable certain API features depending on the version.  

## What are the relevant issue numbers?

Closes #22608, https://gitlab.com/gitlab-org/gitlab-ce/issues/23148

See merge request !6822

Resolve "Move `/licenses` api endpoint to `/templates/licenses`"

## What does this MR do?

It moves the `/licenses`, `/gitignores` and `/gitlab_ci_ymls` API endpoints under the `/templates` namespace

## Why was this MR needed?

In EE we now have somewhat ambiguous API endpoints. `/license` refers to the EE license while `/licenses` (plural) refers to license templates. @DouweM mentioned that we're adding .gitignore templates in #14106 so it may make sense to add a /templates namespace. Then, move the /license templates endpoint to be underneath, along with .gitignore endpoints. 

Closes #17541

See merge request !5717

API: New /users/:id/events endpoint

## What does this MR do?

If add a new `/users/:id/events` endpoint to retrieve a user's contribution events. The events returned are filtered so that only the events for projects that the current user can see are returned (similarly to what we do at the controller level).

## Why was this MR needed?

Because it's a nice feature to calculate leaderboards, for instance for #17815.

## What are the relevant issue numbers?

Closes #20866.

See merge request !6771

The /licenses, /gitignores and /gitlab_ci_ymls endpoints are now also
available under a new /templates namespace. Old endpoints will be
deprecated when GitLab 9.0.0 is released.

API: Share projects only with groups current_user can access

Aims to address the issues here: https://gitlab.com/gitlab-org/gitlab-ce/issues/23004

* Projects can be shared with non-existent groups
* Projects can be shared with groups that the current user does not have access to read

Concerns:

The new implementation of the API endpoint allows projects to be shared with a larger range of groups than can be done via the web UI.

The form for sharing a project with a group uses the following API endpoint to index the available groups: https://gitlab.com/gitlab-org/gitlab-ce/blob/494269fc92f61098ee6bd635a0426129ce2c5456/lib/api/groups.rb#L17. The groups indexed in the web form will only be those groups that the user is currently a member of.

The new implementation allows projects to be shared with any group that the authenticated user has access to view. This widens the range of groups to those that are public and internal.

See merge request !2005

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Memoize Github::Shell's secret token

## What does this MR do?

`API::Helpers#secret_token` was reading the secret file on every invocation. This MR reads the file in the `gitlab_shell_secret_token.rb` initializer and saves it as a class variable at `Gitlab::Shell.secret_token`

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

 - I'm not sure if the use of `cattr_accessor` is the best approach, or if should be moved into the `class << self` block?
 - Should `API::Helpers#secret_token` be removed in favor of using `Gitlab::Shell.secret_token`?

## Why was this MR needed?

Performance optimization.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/22510

See merge request !6599