Allow users to decide if they want entity filling to carry over

[zhujessica]

As of this PR , we assume that if a user has filled an entity for an intent once, we keep the same value for that entity until it is overwritten. We want to add an option where we prompt the user to decide on if they want this entity value to carry over for future intents.

An example where a user might not want to repeatedly fill an entity:
"bake the cookies for 30 minutes at 350 degrees"
"bake the cookies for another 15 minutes" <- in this case, the bake_temperature would carry over from last time

An example where a user might want to repeatedly fill an entity:
"book a flight to [Boston](destination city)"
"book a flight" <- in this case, we would want to ask the user what their new destination city might be