Rename CSS classes related to lazy loading

As mentioned in https://docs.gitlab.com/ce/development/fe_guide/performance.html#lazy-loading

Currently, we have the following:

• when an image is set to lazy load. We add the CSS class lazy to the HTML.
• when an image is lazy loaded. We add the CSS class js-lazy-loaded using javascript.

I propose, we change it to the following:

• From lazy to will-lazy-load
• From js-lazy-loaded to lazy-loaded

We should remove the js- prefix because we've started to use it as a reference in our CSS as seen in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/framework/typography.scss#L24