feat: brand new plugin system 🎉

[did]

• [x] plugin architecture based on Rails engine
• [x] handle custom VueJS code from the engine
• [x] include a generator to generate a new plugin
• [x] refactor the DynamicInput VueJS component in order to receive new custom setting types
• [x] plugins covered by rspec tests
• [x] refactor the way we register setting types in ruby
• [ ] include a generator to generate a new setting type within a plugin
• [ ] write documentation
• [ ] write a default plugin as a test
• [ ] review PR