Create an icon directory

As mentioned here https://gitlab.com/gitlab-org/gitlab-ce/issues/22111#note_16085822, it would be helpful to create a directory of icons used throughout GitLab.

Benefits:

• You would know if there is icon that has two or more meanings.
• Can be use as reference for a specific icon.
• You can determine if that is the best icon for that specific purpose. If not, then switch or change it as in needed.