More i18n automation

It turns out that the LMS team is updating i18n strings by running a paver command, not using Jenkins.  According to @sarina this works because (a) it's relatively low-overhead, and (b) a human being still needs to check validation errors in downloaded strings.

This PR updates our i18n scripts to automate more of the process.  Specifically:

* There are now two i18n commands, "push" and "pull".  The push command was something I was doing manually before.

* I've added a validation step so we can manually verify translation errors.

@stephensanchez This is not *quite* as automated as I'd originally envisioned, but it's a step in that direction.