Translate interface to Spanish

[graue]

It would be great to support common languages spoken in the Bay Area besides English. Spanish seems like the natural one to start with. Later if there's interest we could branch out to the other languages that are included in Muni announcements (Cantonese and Filipino) and beyond.

• basic setup
  • add react-intl
  • select Spanish if it is the browser's default locale
  • allow overriding locale via a query param for dev/testing
  • add formatJS lint rules
• infrastructure
  • create a project on a translation service such as weblate (blocked by Weblate doesn't understand the FormatJS JSON format #248)
  • set up CI to extract and upload messages and download the translations (see pipeline diagram) - This is done except that we have to run the extract and compile scripts manually and commit the results. Should improve later.
• actual translation
  • convert all user-facing frontend strings to translatable messages. Mostly done except:
    • Localize alert messages (e.g. "Can't connect to server") #335
    • Localize bike infrastructure/hill grade descriptions in itinerary #333
    • Localize bike infrastructure labels on map #334
    • double-check that there isn't something else as well...
  • enable formatjs/no-literal-string-in-jsx lint to verify completeness of above task
  • Internationalize numbers we display (24 minutes, 4.8 miles, etc) #336
  • find someone who can translate them for us
• more advanced features
  • setting within UI to change locale