Recommendations using Wikifier model

[stefankreitmayer]

According to Nantes, their LAM recommender API gives the best results when model_type is set to 'wikifier'

Tasks

• [x] fix any remaining bugs using the API
• [x] ensure that all OERs in our db have a X5GON material_id
• [x] set model_type to 'wikifier'
• [x] test briefly
• [ ] show to Nantes and get sign-off