Create admin dashboard widget for rejection stats

[coffee2code]

For evaluative (and potential educational) purposes, it would be handy for moderators to see some simple stats surrounding the frequency of each rejection type. I'm imagining a simple table within an admin dashboard widget that looks like:

Rejection reason	# of rejections	% of all rejections
anothers_art	3	0.77%
branding	4	1.03%
collage	1	0.26%
copyright	19	4.89%
faces	24	6.17%
etc, etc
Total	389	100%

Hovering over the rejection reason should provide the full description of that particular rejection reason since it may not be obvious from the short key.

[coffee2code]

In 14103:

Photo Directory, Rejection: Create admin dashboard widget for simple rejection stats.

• Register and display the "Rejection Stats" admin dashboard widget
• Create count_rejections_per_reason() to return a count of rejections for each reason type.
• Change get_rejection_reasons() to exclude the approval item as a rejection reason by default. It only applies to the dropdown.

Fixes #7796.