Implement skeleton loading up to par with guidelines for Web IDE

Problem

This issue came from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18059#note_66817829

We need to revise how what is loaded in the Web IDE. The current skeleton loading implementations are not up to par with what has been defined in our design pattern library: https://gitlab-org.gitlab.io/design.gitlab.com/components/skeleton-loader

We have sub par skeleton loading in both the content view as well as the sidebar of the web ide

cc: @jramsay @sarrahvesselov @timzallmann