Add mutagenesis

[xwatkins]

Reference to existing issue

• A new endpoint is now available to retrieve additional mutagenesis data. As data was loaded at the "category" level, this meant we needed to now load and transform data at the track level.
• Empty categories were being displayed

Description of changes

• Changed the configuration so that url and adapters are at the track level instead of category level
• Added the new mutagenesis track to the configuration
• Separated data fetching from data manipulation
• Categories are hidden by default, and only shown if they have data to display

Testing/Styleguide

• [x] Unit tests pass
• [x] Type tests pass
• [x] Linters pass

[xwatkins]

Looks good. Function-wise it's working as expected 👍🏽

Fixed an error I found, and moved a bit of the logic around to fix linting and types, so please do have an other look and check that I haven't broken something unexpectedly.

Also, I've proposed a different approach for data loading, but haven't pushed code related to it as it might need further discussion. See related inline comment. This is the only bit that I think might need a change.

Also made a couple of changes after noticing a bug. I had considered using Promise.all but discarded it because of the "fail fast" approach but if there are potential performance improvements thanks to parallelisation happy to give it a go.