add user setting: language

- User.language field
- "settings" API with getter (GET) and setter (POST)
client-side language helper

Language files are served as static yaml files.
language.js exposes functions to load them and "echo" from message code
to actual language's message.
English/Vietnaemse translations for UI
ignore Kate's swap file
copy languages to frontend-dist/static

Because "frontend" dir was ignored in app.yaml, the language yaml files
wouldn't be uploaded to GAE when deploy. Adding a gulp task to move
those files into frontend-dist seems more sensible.
translations for Bookmarked Chapters route
add empty "My Settings" route
remove useless "app" object
simplify "language" module, load user's language

User's language is set in main app component's getInitialState(), right
when user is successfully logged in.

"test-token" API is replaced with "settings" API.
set user's language after logging in
add language setting UI