Fix missing translations for timestamps

I was trying out the Spanish translation (awesome btw!) and noticed that
the snapshot timestamps didn't all have translations. It turns out that
`distance_in_words` has a total of 11 variations, and we only specified
three.

I first attempted at using my high school Spanish to fill in the blanks
myself (hey, I had a an A in Spanish after all). But I realized that I
had no idea what I was doing. Then I came across the rails-i18n gem
which provides default translations for a large number of languages. It
turns out Rails is smart enough to merge multiple locale files together,
so I was able to rip out some stuff from our es.yml.

I did notice that the translations in the rails-i18n for
distance_in_words didn't match the ones that @menor had provided. So I'm
going to ask him to verify that the default es.yml is doing the right
thing:
https://github.com/svenfuchs/rails-i18n/blob/master/rails/locale/es.yml