Internationalize dates and times in UI

[gnprice]

In a few places in the app we show a date or a time — most notably in the message list for when a message was sent. (Currently just the dateless time on each message; soon dates in #172 and #173.) Currently these are in a constant format that doesn't respond to the user's locale. We should internationalize them.

This will require #275.

This is a relatively late priority because it's something zulip-mobile doesn't do; see zulip/zulip-mobile#1529 and zulip/zulip-mobile#4814. OTOH Zulip web does accomplish this, at least in the most visible spots: the right column of "Recent conversations", and the times and dates at the right edge of the message list.

[gnprice]

Another place dates and times appear in the UI is the lightbox:

• lightbox: Use a friendlier format for the date, like "Yesterday at 4:47 PM" #45