i18n logistration

This is i18n of all things logistration. I've included translator's comments where possible to make the usage of strings clear. As far as myself, Sarina, Andy, and DB know, there's no good way to include translator's comments in our Underscore templates; I'm open to suggestions on this.

@AlasdairSwan and @dianakhuang, could one of you take a look at this when you have a moment?
One way of doing this is to ask the server for the platform name, and then use that throughout the templates. We can also drop "edX" from the copy; that would leave this line as "I am a returning user," which actually mirrors the "I am a new user" text nicely. @AlasdairSwan and I are both inclined to go with the latter approach for now.