These reject/approve buttons were getting `btn-default` classes rather
than `btn-danger` or `btn-success`, which meant the styles for the
colors had to be applied separately to `.approve-post` and
`.reject-post`. This also meant the styles needed an additional override
in themes.
Moving to our common button classes helps centralize the styles and
avoids a manual override being needed in themes.
Button appearance is the same before/after
