Install Generator to Properly Install Engine in Host Apps

[adrianna-chang-shopify]

Closes: #16

Background

• We want users to be able to easily set up their host app for use with our gem
• Providing an install generator is IMO the best option, and can handle mounting the engine, running the migrations, and creating our initializer files (it's also the way I've seen other engines handle installation, such as devise)
• Users will simply need to install the gem, and then perform rails g maintenance_tasks:install in order to complete the setup

Summary of Changes in this PR:

Implementing abstract_class in MaintenanceTasks::Task

• Needed so that we exclude ApplicationTask from the list of tasks that are available to enqueue in the UI
• Will allow users to specify other groupings of tasks if they'd like, and have them excluded from the UI

Implementing the install generator

• Generator will mount the engine, copy and run migrations, create an initializer for our engine as well as a base ApplicationTask class
• Testing the generator is a bit tricky because it requires a sample app to be set up to do things like mount the engine and run the migrations. To achieve this, I've basically copied the dummy app into a sample directory, and removed things we don't want. This test is slow because it needs to run db migrations. I think we need test coverage for this, so I'd argue that one slow test is manageable

To 🎩

• Navigate to http://localhost:3000/maintenance_tasks

To test the install generator:

• cd test/dummy
• rm db/schema.rb config/initializers/maintenance_tasks.rb app/jobs/maintenance/application_task.rb
• rails db:drop (so we can run the migrations)
• rails g maintenance_tasks:install
  AFTER YOU'RE DONE
• Remove the copied migration and recreate the db rails db:drop db:setup

[etiennebarrie]

Really nice!
We should also add that to the README so that people know to run that after adding the gem.

[adrianna-chang-shopify]

@volmer this is ready for a look when you have a chance 😄