Add Basic Avatar rules

References #1 (closed)

Adds basic semgrep rules and tests for GlAvatar & friends.

On current latest master commit

semgrep --severity WARNING --config rules/components/avatar /Users/olenahoral/projects/gitlab-development-kit/gitlab/{ee/,}app

yields 0 results

where

semgrep --severity INFO --config rules/components/avatar /Users/olenahoral/projects/gitlab-development-kit/gitlab/{ee/,}app

gives me 81 results

I believe the search for deprecated avatar implementations should be improved by searching for things like

= image_tag avatar_icon_for_user(@user, 60, current_user: current_user), class: "avatar s60"

or

= user_avatar(user: user, size: 40, user_name: user.name)

but that would be the next steps