Add SSR CSS skeleton loader for the project page navbar

What does this MR do and why?

For the POC added a SSR Skeleton loader for the project page sidebar.

The CSS animation is similar to our skeleton loader's (thanks @kpalchyk for helping with it). SVG animation does not work while content is loading gitlab-ui#2090 (moved)

Screenshots or screen recordings

Just animation

When loading the page

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #383098 (closed)