[WIP] Add language picker

This also contains the changes in #1912, so the diff should simplify a bit once that's merged.

Some commentary inline on the remaining rough spots.
`currentPath` gets used by `webmaker-language-picker` to build a link to function as a fallback if `.ready()` isn't called, I think. The link looks like `href="/<%=localeInfo.lang%>/languages?page=<%=currentPath%>"` which I don't think makes sense for Appmaker, so maybe that should be made customizable somehow.