Sweep through UI strings to internationalize

[gnprice]

This is a followup to #275. After we have that basic framework for using translated strings in the UI, we'll want to sweep through our existing code and convert it all to use that framework.

Initially this won't do much of anything from a user's perspective, because we won't have more than a handful of strings translated until we wire things up to a service where people can contribute such translations, #276.

Nevertheless it's useful to do this early, because it sets the pattern so that we naturally follow it in new code, rather than having more code that needs to change in an eventual sweep later. We may also come across places that need a context object and didn't before, or otherwise call for refactoring, and it's convenient to do that sooner so that other changes to the same code are based on the new structure from the beginning.

Other notes:

• Many of these spots are marked with TODO(i18n), though many others are not.
• Some spots marked TODO(i18n) are for different needs like formatting dates and times. Those are out of scope of this issue.
• If some strings are especially difficult to wire up for translation for some reason, we can mark them with TODO(i18n) and also file followup issues, and then leave them out for this issue.