Improve LMSReferenceItemWidget

Problem/Motivation

Currently we have a minimalistic version that mostly uses core API, it'd be nice to have something more user-friendly.

Proposed resolution

Tabledrag in expandable details element. Columns:

1. Entity - not editable, just the entity (Activity or lesson) name
2. Additional parameters, each in a separate column (Max score if it's an activity in a lesson or mandatory and min success score if it's a lesson in a course) - editable
3. Delete button (AJAX)

Add button below the table (there can also be one above if there are many activities) - opens modal dialog with a form: entity autocomplete, parameters, submit (when clicked and validation passes modal is closed and entity is added to tabledrag); also create new button -> multistep form in the same modal:

1. Entity type selection if creating a new Activity (Lessons don't have bundles)
2. Entity creation form

Remaining tasks