The "Show extra whitespace" button in the diff viewer was defaulting to

the wrong state. This should be bound by a cookie, but that was being

ignored, causing it to claim that showing extra whitespace was off by

default. In fact, clicking this would toggle the cookie's value, easily

causing mismatches.
This could have been an easy hacky fix, but in reality, our buttons here

are pretty bad. Most buttons had two versions in the template: An active

state button, and an inactive state button. We'd use CSS to show or hide

the appropriate button, but this meant that under the hood there were

still two buttons being exchanged.
This change cleans all this up, helping future-proof this and improving

both usability and accessibility. The template no longer hard-codes any

buttons. Instead, these are built via JavaScript, based on state passed

down to the page's model.
Each conceptual button has only a single actual button on the page (as

a <button> and not an <a> tag). The properties of that button are

modified when clicked. When registering, the active/inactive text and

a description (for tooltips and screen readers) are added. The callback

is added at the same time. This keeps each button manageable. It also

theoretically enables extensions to add custom buttons (though this

won't be formally documented).
While this change is being made for Review Board 4, a backport to 3 will

be in the works for a customer affected by this issue.

Tested the functionality of all buttons, ensuring they reflected cookie

state.
Verified the buttons still look the same (in terms of presentation) as

they did before.
Unit tests passed.