Clean slate installs and the jumpstart:override generator

New Jumpstart Pro apps now start with a clean slate. Most of Jumpstart's models, views, helpers, mailers, and Madmin resources have moved into the Jumpstart engine, so your app/ folder only contains your own code.

This means far fewer merge conflicts when you pull in upgrades, since you're no longer editing Jumpstart's files directly.

When you do want to customize something, copy it into your app with the new generator:

bin/rails generate jumpstart:override app/views/path/to/file.html.erb

We also trimmed dependencies along the way: the inline_svg and oembed gems have been replaced with lightweight built-in implementations, and the unused fa_icon and language_options helpers have been removed. See UPGRADE.md for generator and diff examples.