Add live stream icon

Purpose

The live-stream icon exists in the UI as an individual SVG, but this is to include it as part of the SVG library. See discussion below regarding some design improvements that are coming along for the ride.

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 #317 (closed)