Implement proper caching of Airtable requests (add state management?)

[tuttiq]

Is your feature request related to a problem? Please describe

The current caching logic doesn't work, so we're still making requests to Airtable to get the full list of prefectures, languages and topics every time we load the speaker list and the nominate form.

Since we shouldn't have any more N+1 requests problem, it's a not a huge deal right now, but could improve the performance slightly and more as we add more pages to the site in the future (speaker profile page, etc?).

Describe the solution you'd like

I think we should probably just add centralized state management with Vuex.

Describe alternatives you've considered

I tried a few workarounds for caching, but they all have problems (serializing/deserializing data, etc).