Refactor TrendingProjectsFinder to support caching

## What does this MR do?

This refactors `TrendingProjectsFinder` so it can support caching of the data. See cb7d398972d786ba7133418266fa34ae641b2497 for more details.

## Why was this MR needed?

Trending projects is quite slow, easily taking seconds to load the entire page.

https://gitlab.com/gitlab-org/gitlab-ce/issues/22164

https://gitlab.com/gitlab-com/infrastructure/milestones/4, in particular the section "Trending page under 2s"

See merge request !6672