Store the last completed unit node and pass it to the backend

[milsen]

Node pair cost functions calculate cost values for pairs of unit nodes. When a recommended learning path is computed for a node pair cost function, it has to be determined which unit node to start with.
Intuitively, this should be the node v with the lowest cost value for the node pair (u,v) where u is the last completed unit node.
Currently, the last completed unit node is not stored and not passed to the backend. The backend always uses a heuristic to determine the node to start with.

Thus, it may occur that the learning path changes unpredictably while completing the unit nodes it contains. Example:

1. The recommended learning path is 1 -> 2 -> 3.
2. The learner completes 1.
3. The new learning path is 3 -> 2 (because the first node in the learning path is chosen heuristically rather than as the node v with the best cost value for (1,v)).

The last completed unit node could be stored in the mooc_fields table:

• block_id: Id of the last completed block
• user_id: Id of the respective user
• name: 'last_completed'
• json_data: empty (or true or some placeholder value)