Normalize programme information in index

[dcabo]

We currently store the programme id, title and date in each caption. Normally it works well, Elastic prefers to have things denormalized, but it's starting to be painful when we get a list of programmes in the database, which seems to be taking around one second now.

The solution would be to define a parent-child relation in the index, as explained here:

• https://www.elastic.co/guide/en/elasticsearch/reference/current/parent-join.html
• https://www.factweavers.com/blog/join-in-elasticsearch/