Tech debt: Create bundle for static pages for CSS and JS

In our error pages we have a lot of repeated code, see this MR !9017 (merged)

We should do the following:

I imagine having static-pages.css and static-pages.js and they are both included in static pages. C/P'ed CSS and JS codes can live in those files. I think we don't need WebPack to do that. Just create two static files and do some cleanup, that should work I think.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9017#note_22953073

//cc @fatihacet