Add language support

[PurkkaKoodari]

Add support for language versions of events.

The app can be used like most guilds, including Tietokilta, are currently doing: include Finnish and English in the same description field. However, we'd like to implement distinct language versions for each event.

• The following must be localizable:
  • Event title, description, location, price, website URL, Facebook URL
  • Category (this may be reconsidered, as these should be kept in sync between locales)
  • Quota names
  • Questions
  • Answer options
• The system might work as follows:
  • The app has a list of allowed locales, likely based on the available UI strings.
  • Each event has a "default" locale (will be migrated from the previous contents).
  • In addition, the editor allows creating additional locales with a specified locale.
• The language shown should be the same as chosen for the UI strings (Add internationalization for UI strings #93), both in the app and integrated components.
  • If the event does not have that locale, the default locale will be shown instead.
• Creating a locale must be easy and would likely involve a way to copy values from the default locale.
• There should be an easy way to update all locales at once.
  • For example, when a field has the same value in multiple locales, and is modified in one, we could automatically update it in all locales or prompt the user to do so.
  • Alternatively, we could have an option to mark fields as "use in all locales" (in the default locale) or "copy from default locale" (in other locales), and that would be enabled by default.