Promote "recommended" feature-complete themes

Description of the need

We need to make it easier for people to find the themes that are the most feature-complete, and the most likely to look great immediately when installed.

Proposed solution

If we could come up with some criteria for what a "feature complete" theme is, we could review the themes we have available, and promote the ones that meet this criteria as "recommended".

To start, these could be the same way as we treat "recommended" modules on the installer page, but we could also promote them directly on the Appearance page.

TODO: * Determine what makes a theme "feature complete" (see below for a 1st draft) * Evaluate current themes for "feature complete" ness, and promote these themes on b.org * Highlight these themes in Backdrop itself

We will need to nominate a team of people who are responsible for reviewing themes, and/or a process for people to submit new themes for review.

A feature complete theme must include: 1. Support for all 10 core layouts 1. Style for all elements present in the Styleguide module 1. Style for teasers as well as full-content nodes 1. Style for some basic views displays, including a) teaser view 2) list view 1. works both with and without a logo

Follow-ups: * [ ] Add in issue template to the backdrop-ops/contrib queue for promoting a theme w/ the checklist