Prettier code folding elements

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Currently, we have these little boxes for code folding:

I think we can make this look prettier. For example, GitHub uses these blue lines to control the folding:

They are defined in src/components/diff_view.rs in the DiffLineGroup component, and their style is in static/style.css.

[xfbs]

Idea: can use the iconography of GitHub for this, since they permissively license them: octicons. GitHub has put a lot of thought into creating these icons, they are pretty and coders should be used to them.

[xfbs]

I've just pushed a commit which implements this.

Currently broken:

• line numbers for old/new aren't right, needs fixing
• want to add a button expand all
• github does an incremental unfold.. when you press the button, it reveals the next 15 lines, but not all of them. not sure if we want this too or if we want to keep it simple.