Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/24287

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1720

After https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10563, and while waiting for the build for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1556 to succeed, I couldn't help myself.

We now have:

• a totally refactored backend for BlobViewers,
• a Source|Rendered switch that shows "Rendered" by default, except when the URL anchor starts with #L, which means line number anchors now work to link to specific lines in Markdown source!
• a file size limit of 10MB on client-side rendered files, so we don't force users to download huge files,
• and a .blob-viewer[data-type=simple] that rich JS viewers can fall back to when the rich rendering fails for whatever reason. They should hide() their own blob viewer (.blob-viewer[data-type=rich]) and the switch buttons (.js-blob-viewer-switcher), and show() that simple fallback viewer, which will be the regular Download or Text view.

To do:

• Split up in a sensible way
• Specs

The switch can be seen in action with SVG, but also supported for Markdown and IPython Notebooks, as well as other supported Markup formats:

/cc @rspeicher @jschatz1 @rymai @grzesiek