Showcase underrepresented grants

### Why Is this Needed

Some grant owners may purchase ads to show their grants more often than others. That creates inequality in representation. That's bad.

Here I present a way to mitigate this problem

*Summary*:

### Description

*Type*: Feature

### Expected Behavior

When showing a grant, also show previews of several other (related? random topics?) grants, with emphasis on grants that both:

- have low view counts (as determined for example by Google Analytics, because it has powerful spam filters if they are turned on)
- have good view characteristics (low bounce rate, high session duration, and high "percent of donations" (either of their number or of the amounts))

Also show such grants after a donation to any grant and featured in grant lists (and create a special "Underrepresented" sort order of grants).

### Data Requirements
Measuring is hard, but we can seek to reduce the quadratic deviation of views.