Add Header text "Before | After" above diff-card

[thadguidry]

Problem: It's hard to distinguish with only color and small -/+ symbols in the diff-card to determine which side is Before and which side is After a change for new folks.

Possible Solution: Somewhere here above diff-card should be added a Header div or just simple Text labeling in a <div> or <span> to distinguish which side is Original | Changed or Before | After. https://github.com/google/wikiloop-battlefield/blob/master/components/RevisionCard.vue#L87

Example:

Before	After
Line 316:	Line 316:
{{s-off}}	{{s-off}}
{{s-par	it-lwr}}
etc.	etc.

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[xinbenlv]

Good idea, thank you