Improve pipeline triggers API

What does this MR do?

This introduces /api/v4/projects/:id/trigger/pipeline that replaces /api/v3/projects/:id/trigger/builds.

Previous build trigger was user-less, the new pipeline trigger is user-aware. This MR introduces backend changes to make it user aware. The new create token with API will have a user assigned.

This MR also introduces trigger description and slightly improves API:

1. POST /api/v4/projects/:id/triggers - creates a trigger with permission of current user and requires description,
2. PUT /api/v4/projects/:id/triggers/:trigger_id - updates a trigger description,
3. POST /api/v4/projects/:id/triggers/:trigger_id/take - takes ownership of trigger,
4. POST /api/v4/projects/:id(/ref/:ref)/trigger/pipeline - returns Entities::Pipeline instead of Entities::TriggerRequest

This also contains a part of changes that are introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9277. And it's there to solve this story: https://gitlab.com/gitlab-org/gitlab-ce/issues/27156.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?