Add new dictionary_item field: introduced_in_skill

[Skivling]

I remember reading issues where all possible options from the whole dictionary were shown in the tooltips, including a lot which hadn't been introduced yet.
I propose a field in each dictionary_item which identifies the skill that created the dictionary item by the uuID. Then applications could choose to only show options from skills that the learner has begun.

In a hypothetical dictionary view page, an application could then hide non-introduced definitions or have them slightly hidden.