Don't show 'Copy content' button on text files that are not rendered as text

Bug introduced in !9878 (merged).

The "Copy content" button reads the content to copy from the DOM, and can only do that when the actual blob/_text partial is displayed, and not if the textfile is rendered as, say, SVG or Markdown.

The large amount of changes was "necessary" to make sure Blob#to_partial_path actually only returns text when it's actually going to render as text.