Migrate `app/views/shared/_confirm_fork_modal.html.haml` modal implementation to `GlModal`

The markup in app/views/shared/_confirm_fork_modal.html.haml replicates that of GlModal. It should be replaced with the implementation using GlModal. Modal display is handled by app/assets/javascripts/vue_shared/components/web_ide_link.vue

Migration tips can be found in parent epic.