[Refactor] migrate all assets to webpacker (JS CSS and images)

• https://github.com/thoughtbot/administrate/wiki/Rails-6-&-Webpacker > Follow this
• move stylesheets
• move images
• compare new bundle size to old one (cf below)

change list:

• all CSS, JS and images moved from app/assets to app/webpacker
• stylesheet_link_tag becomes stylesheet_pack_tag
• image_tag becomes image_pack_tag
• for SCSS packs to be created, they NEED to have a JS pack associated to it.
• FontAwesome upgraded from 4 to 5:
  • some icons changed names
  • old icons with -o suffix like .fa.envelope-o become .far.envelope
• print stylesheet was included independently in the app/views/common/_meta partial, I've moved it to be imported and bundled together with the main application stylesheet. it's tiny
• explode require 'rails/all' into separate requires in order to exclude sprockets (even though it still seems to be loading

upon releasing :

• extensive test of the app on demo:
  • fonts
  • all layouts:
    • administrate
    • landing
    • application
    • application_users
    • mail
  • images, images inserted through CSS, favicon etc
  • FA icons
  • js interactions
    • timepickers
  • recheck lighthouse score (current on demo: 73)

comparison of bundle sizes before and after seems constant (but way too high) for application and landing packs (done using SECRET_KEY_BASE=test RAILS_ENV=production rails assets:precompile on both master and this branch):