Move all static frontend fixtures to static directory (EE-port)

EE-port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27668

What does this MR do?

Move all static frontend fixtures to the static directory.

part of https://gitlab.com/gitlab-org/gitlab-ee/issues/10675

Before

$ git ls-files spec/javascripts/fixtures/ | grep -v '\.rb$' | grep -v '/static/'
spec/javascripts/fixtures/.gitignore
spec/javascripts/fixtures/images/green_box.png
spec/javascripts/fixtures/images/red_box.png
spec/javascripts/fixtures/one_white_pixel.png
spec/javascripts/fixtures/projects.json

After

$ git ls-files spec/javascripts/fixtures/ | grep -v '\.rb$' | grep -v '/static/'
spec/javascripts/fixtures/.gitignore