Add screenshot.png image to install profiles and show them in the installer

Problem

• Even the best labels and descriptions cannot fully clarify what a certain install profile will look like after installing.

Goal

• Depict how a site looks right after installation.

Proposed solution

1. Add screenshot.png image file to each installation profile.
2. Display the screenshots for each install profile on the installer's profile selection page. (copy/paste markup + CSS from "Appearance" theme selection/administration page)
3. Profit.

Relates issues

• #1664894: Clarify Minimal profile's use-case and description →