* FEATURE: Create rake for db migration in plugins
before the dev-xp was clunky, we had to create a migration file in core and
move it to the plugin.
Now we automated this process, we still create the migration file in core
but the rake task will move it to the plugin.
the usage is:
```
rake plugin:generate_migration[plugin_name,migration_name,migration_args]
rake plugin:generate_migration[discourse-automation,add_group_id_to_automation_rule,"group_id:integer"]
```
* DEV: change rake to be a generator for plugin migrations
* DEV: trying to add extra class option to migration generator
* DEV: revert to have only `plugin_migration_generator`
* DEV: remove rake task for plugin migration creation
* DEV: remove migration_generator.rb
* DEV: remove if because options with `plugin_name` will always be true
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