Merge Single and multiple choice plugins into one configurable

Problem/Motivation

Now that we have the ability to configure Activity - Answer plugins, we can have one selection plugin with single / multiple choice configuration instead of two serving the same purpose.

Proposed resolution

1. Merge the two plugins into one plugin class.
2. Add an update hook that will change the plugin and add relevant configuration for existing activity type entities.
3. Update QA data in /tests/data: add activity_types.yml file with configured QA activity types used in automated tests (all plugins except the feedback ones, 2 types for the merged plugin with 2 possible single / multi choice configuration option).
4. Update Drupal\lms\LmsContentImporter::installActivityTypes() to use the config from the activity_types.yml file.
5. Update automated tests (Drupal\Tests\lms\FunctionalJavascript\GeneralLmsTest::adminTest()) to use configuration from the activity_types.yml file when creating activity types.

User interface changes

Instead of two plugin options (in addition to other plugins) when selecting a plugin on activity type add form we'll have only one but with a configuration of single or multiple choice.

Remaining tasks