refactor(locales): move to using static file

[LoneRifle]

Problem

We want to use a single codebase for several similar deployments, but
with different themes and copywriting text. To that end, we need resource strings
to be easily substitutable. As of now, they are implemented as a TypeScript module
webpacked into the bundle

Part of work for #777

Solution

Move translation resource strings to static file, so that we can easily
substitute such files for deployments using the Go.gov.sg codebase

• npm install i18next-http-backend
• app/assets/.../translation -> public/locales/.../translation.json
  • Make use of i18next's in-built nesting feature in lieu of the
    string interpolation were doing in the app/assets ts file
• add i18next-http-backend to i18next bootstrap, so that we fetch
  translations from a static path instead of having it injected into
  the config
• wait on i18next initialization before proceeding to render
• FeatureListSliver - swop out <Trans> elements for i18next.t calls

Deploy Notes

New dependencies:

• i18next-http-backend : backend that fetches translations from a defined path

[yong-jie]

looks like the <Trans> is also used in LandingGraphicSliver.tsx. Do you also plan to migrate that to vanilla i18next? we might be able to remove react-i18next if that's the case

[LoneRifle]

looks like the <Trans> is also used in LandingGraphicSliver.tsx. Do you also plan to migrate that to vanilla i18next? we might be able to remove react-i18next if that's the case

<Trans> is being used here to honour the style tags that are found in the resource strings.