Typesafe i18next keys

[JGreenlee]

After we have removed Angular, we should explore the possibility of having the t function of i18next be typed according to en.json. https://www.i18next.com/overview/typescript
This way, we could ensure we never try to access a key that doesn't exist. It would also work with IntelliSense to give us autocompletion for existing keys.

We would need to:

• upgrade to latest i18next and react-i18next
• enable strictNullChecks in our tsconfig.json (a good idea anyway and more achievable now than several months ago)
• add a file called types/i18next.d.ts:

import enJson from '../../i18n/en.json';

declare module 'i18next' {
  interface CustomTypeOptions {
    resources: { translation: typeof enJson };
  }
}

[JGreenlee]

This was implemented and is on master now