Page %Util Histogram

## Overview

Currently it is difficult to understand what causes the page utilization to be what it is. It's also difficult to understand the utilization of more than one page at a time.

## Changes

This pull request includes several changes:

1. Shows the %util for every child page when viewing a branch page.

2. Shows the %util for the root page of every subbucket when viewing a leaf page.

3. Adds a link to display a histogram of all page in-use values at and below the current page.

## Notes

The histogram uses the square root choice method to determine the number of groups to show. The number of groups can get very large for low-depth pages on large DBs so this number is capped at 100.

Here is calculation:

> min(100, ceil(sqrt(subpage_count)))

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/cc @snormore @mkobetic