Modernize repo to latest everything, to run on Heroku again

[veganstraightedge]

Demo running on Heroku

https://temp-mf-rb-b59ec22d01d4.herokuapp.com

Screenshots

(at the bottom of description)

Dependencies

• Ruby: 3.2.2 → 3.4.5.
• Rails: 5.2.0 → 8.0.2
• Gems: everything → latest
• Bootstrap: 4.1.3 → 5.3.7
• Rubocop: 0.58.2 → 1.79.2
• Contributor Covenant Code of Conduct: 1.4 → 3.0

GitHub Action CI

• Added Dependabot to keep gems up to date
• Added Rubocop PR check
• Added ERB lint PR check
• Added Brakeman PR check

Code changes

• Rubocopped all of the things
  • Generated a .rubocop_todo.yml for the last TODO to make (a db schema change)
  • Added Rubocop extensions for: Rails, Rspec, Capybara
• Updated everything to modern Rails-isms
• Removed unneeded performance profiling and dev gems
• Renamed submissions/_form.html.erb to submissions/_submission.html.erb, so render @submission pattern works
• Removed the extra hidden field values from submitted form URL, using enforce_utf8: false
• Added various binstubs, for CI and dev

Assets (Bootstrap)

• Removed the asset pipeline related Bootstrap gems
• Removed Bootstrap JS (no JS needed)
• Linked to Bootstrap CSS from CDN directly
• Removed the few CSS customizations
• Made the design calmer (less boldness: headings, labels, etc)
• Added dark mode, and auto switcher which uses the system's settings
• Linted all of the HTML
• Added a helper with constants to more easily update Bootstrap version
• Updated some Bootstrap class names
• TODO: Someone should add a good microformats logo SVG as favicon

module ApplicationHelper
  BOOTSTRAP_VERSION       = '5.3.7'.freeze
  BOOTSTRAP_CSS_INTEGRITY = 'sha384-LN+7fdVzj6u52u30Kp6M/trliBMCMKTyK833zpbD+pXdCLuTusPj697FH4R/5mcr'.freeze

Tests

• Removed the routing/view/controller tests which were testing implementation internals, not user experience external
• Added Capybara system tests, which actually load and click around / submit forms in a (headless) browser (Chrome)

Screenshots

Light mode

Dark mode

[jgarber623]

The diff here is too big for me to thoroughly review, but I did a quick test of the website at the link @veganstraightedge provided in the description and it appears to work just fine!

Tested with a URL and with an HTML snippet and validated that the results were as expected.

[veganstraightedge]

@aaronpk et al— can you review and merge this pls?