Small fix ups for memory size outputs in UI

This issue goes in the 'polish' category. On regionserver ui, we were listing raw bytes for heap size, memstore size, etc. I put in place StringUtils.humanReadableInt (looked to see if bootstrap could do it for us but doesn't seem so, not w/o plugin). I then made all the megabytes and kilobytes match StringUtils.humanReadableInt with its 'm' instead of 'MB' and 'k' instead of KB. Removed a stray KB that was in the wrong place too.