Styling for Apply/Discard buttons

[rtfeldman]

WIP - do not merge; not fully wired up yet!

Change the "Apply" and "Discard" buttons to match @danilo-leal's design!

Visual notes for @danilo-leal

• The button container is taller than the editor line height, so for single-line diff hunks it extends down below the bottom of the hunk. I think we should make the font size smaller to avoid this
• I think we should try a design where we keep the icon buttons until the cursor touches the hunk, at which point we expand to the spelled-out words "discard" "apply" with the keyboard shortcuts. I think when many hunks are visible, the words in the buttons take up a lot of real estate.

Implementation todos:

• make the buttons expand based on the cursor position
• fix the hunk header block preventing clicking on the first line of the diff
• hide the undo button for now

Release Notes:

• Restyled Apply/Discard buttons