New product icon > thumbtack-solid

Purpose

Adding a solid state for the thumbstack icon to accommodate the feature to add pinning in groups an projects in gitlab#378547 (closed)

Screenshots

thumbtack	🆕 thumbtack-solid

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 #354 (closed)