Fix Image Assets After Upgrade to Phoenix 1.6

[kinson]

Overview

This pr fixes a few issues that I noticed after the upgrade to Phoenix 1.6, including:

• fixes an issue introduced in #140 where image assets were not loading
  • before: https://elixirschool.com/blog/instrumenting-phoenix-with-live-dashboard
  • after: https://school-house-pr-147.herokuapp.com/blog/instrumenting-phoenix-with-live-dashboard
• fixes an issue where styles inside heex templates were not being applied (by adding heex to the purge list in tailwind.config.js`)
  • before: https://elixirschool.com/en/conferences
  • after: https://school-house-pr-147.herokuapp.com/en/conferences
• adds the favicon to the site 💅🏼
• updates the .gitignore file to ignore the whole priv/static directory