Add a sidebar icon

Purpose

Adding a new "sidebar" icon to use to toggle a sidebar instead of the hamburger menu.

Screenshots

Checklist

For a new icon and/or illustration:

• Link to related design issue:
• If this adds an icon ensure that it follows the new icon checklists.
• If this adds an illustration ensure that it follows the new illustration checklists.

For third-party trademarks:

• If this adds or updates an icon or illustration that includes a third-party trademark, ensure that it follows the process for adding third-party trademarks to GitLab.

Closes #313 (closed)