Build transformers for each content types

[rcantin-w]

Build transformers as per transformed documents results in the "All" search RFC

• [x] Books https://github.com/wellcomecollection/content-api/pull/163
• [x] Events https://github.com/wellcomecollection/content-api/pull/169
• [x] Exhibition texts #171
• [x] Exhibitions #172
• [ ] Highlight tours
• [ ] Pages
• [ ] Projects
• [x] Seasons
• [x] Stories https://github.com/wellcomecollection/content-api/pull/170/
• [x] Visual stories https://github.com/wellcomecollection/content-api/pull/164
• [x] Parent transformer file that sends each document through its relevant transformer https://github.com/wellcomecollection/content-api/pull/162
• [ ] Filter out delisted (scheduled) events before they reach the transformer