Repo editor: Replace FontAwesome Icons with GitLab Icons

The icons currently being used in the Repo Editor are FontAwesome icons. These need to be replaced with unique GitLab icons per https://gitlab.com/gitlab-org/gitlab-ce/issues/32894

cc/ @hazelyang @tauriedavis

changed the description

added Platform label

mentioned in issue #37808

added multi-file editor and removed ~2458195 labels