Create, Build, and Style > Tree

This issue is to complete the ~"pajamas::create", ~"pajamas::build", ~"pajamas::style" portion of ~"component::tree" as part of FY20-Q2 OKR to Improve the quality and consistency of our UI.

This includes a single-source-of-truth component that is implemented in Vue and thoroughly documented on design.gitlab.com. This does not include implementing the component throughout the product.

Related epics:
*  [Components of Pajamas Design System](https://gitlab.com/groups/gitlab-org/-/epics/973)
*  [Pajamas component: Tree](https://gitlab.com/groups/gitlab-org/-/epics/1124)

Related issues:
*  [FY20-Q2 UX OKR: Improve the quality and consistency of our UI](https://gitlab.com/gitlab-com/www-gitlab-com/issues/4355)

## Checklist

* [x]  Usage documentation is included in Pajamas
* [x]  Design specs are included in Pajamas
* [x]  Component is built and styled in gitlab-ui according to specs
* [ ]  Banner on vue tab that warns of incorrect styling is removed from Pajamas
* [ ]  All demos and todos are complete
* [ ]  Status is changed to `Built` in Pajamas
* [ ]  Add a reference and link to the completed component in the [Engineer Week in Review](https://docs.google.com/document/d/1Oglq0-rLbPFRNbqCDfHT0-Y3NkVEiHj6UukfYijHyUs/edit) doc

### Checklist for UI/sketch updates

Make sure these are completed before closing the issue, with a link to the
relevant commit or issue, if applicable. Get familiar with the [Sketch UI Kit
documentation](https://gitlab.com/gitlab-org/gitlab-design/blob/master/doc/sketch-ui-kit.md)
which has information on updating files, structure, fonts, and symbols.

1. [x] **Author**: Create a Sketch file in your progress folder with the
   changes required for this issue. Try to use existing symbols, layer styles,
   and text styles.
1. [x] **Author**: Ask another Product Designer to review your personal Sketch file, linking them to your latest commit so they know where to find it. If they have the capacity, they should assign themselves to this issue. If not, try pinging another person.
1. [x] **Reviewer**: Review and approve author's changes in their personal
   Sketch file, according to the [workflow](https://gitlab.com/gitlab-org/gitlab-design/blob/master/doc/sketch-ui-kit.md#sketch-workflow).
1. [x] **Author**: Add your changes to the GitLab Sketch UI Kit (pattern
   library and/or instance sheet), following this [step-by-step process](https://gitlab.com/gitlab-org/gitlab-design/blob/master/doc/sketch-ui-kit.md#when-changes-are-approved).
1. [x] **Author**: Ask the reviewer to review your changes to the Sketch UI
   Kit files.
1. [x] **Reviewer**: Review and approve author's changes to the Sketch UI Kit
   files, according to the [workflow](https://gitlab.com/gitlab-org/gitlab-design/blob/master/doc/sketch-ui-kit.md#sketch-workflow).
1. [x] **Author**: [Create an issue in the Design
   System](https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/issues/new)
   to update the design **specs** and **documentation**. Mark it as related to
   this issue.
1. [x] **Author**: Add a read only (FYI) agenda item to the next [UX weekly
   call](https://docs.google.com/document/d/189WZO7uTlZCznzae2gqLqFn55koNl3-pHvU-eVnvG9c/edit?usp=sharing)
   to inform everyone of these changes, linking to this issue.

Story points: 3.0