Improve cache ergonomics

[CohenArthur]

This PR does multiple things as I was stuck on a train without internet:

1. Add an /api/dates route to indicate all dates recorded in the cache
2. Add ways to write cache to disk
3. Add ways to load cache from disk
4. Add mocking to test the cache even when making requests is not possible (heh)

[CohenArthur]

This still has a few issues:

• Does not save cache data properly (file names are missing a date)
• Does not try to update the cache from the local cache properly (does not update its latest date etc)

[CohenArthur]

Closing since #32 has been merged