Image upload with ActiveStorage

[wvengen]

• Prototype
• Make it work with and without importmaps
• Detect when to enable ActiveStorage and when not (improve detection), see also ActionText detection in app/assets/javascripts/rails_admin/application.js.erb produces false positives railsadminteam/rails_admin#3659
• Use url helpers (or alternative) to generate ActiveStorage paths, when possible
• Allow enabling/disabling with option
• Let TinyMCE work with absolute paths (it translates it to a fully relative path, which breaks rendering in other contexts). Or use an absolute URL (but that is less portable, not preferred).
• Provide guidance / helpers on coupling uploaded images to the model (so model deletion also removes the images).

[wvengen]

ActionText gets the image (preview) URL by including a URL template for uploaded images here: https://github.com/rails/rails/blob/main/actiontext/app/helpers/action_text/tag_helper.rb