Make loading_icon helper conform to GlLoadingIcon

This renames the `loading_icon` helper to `gl_loading_icon`, and makes
its API and output more similar to the `GlLoadingIcon` Vue component:

- The container element is now always rendered.
- The container element is a block-level element by default.
- The `container` argument has been replaced with with `inline`,
  defaulting to `false`.
- The container element now has `role="status"` for better
  accessibility.
- The default colour of the spinner is `dark`, as `orange` is not
  a valid colour (see
  https://gitlab.com/gitlab-org/gitlab-ui/-/issues/1029#note_431080670).

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/352510, to aid the
migration described in
https://gitlab.com/groups/gitlab-org/-/epics/7628.