Unify file previewers

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We are introducing a number of file previewers. These all have (or rather, can have) a set of common methods that should be worked into a base class that can be extended.

Common behaviours:

• Parse something with some imported parser
• Render something with some renderer
• Start spinner whilst parsing, stop spinner when rendering
• Insert large previews with minimal perceived performance impact (lazy insertion)
• Throw up a Flash error for previews that are too large and revert to raw download UI
• Raw|Rendered toggle button gitlab-ce!10566
• Advanced/Strech: Run parser on a web worker that communicates fluidly (stream-like) with the lazy document renderer.

I miss anything?

/cc @jschatz1 @iamphill @samrose3