R1:ccc7c1b42436

Make i18n string extraction faster and more flexible

Summary:
Ref T5267. Two general changes:

- Make string extraction use a cache, so that it doesn't take several minutes every time you change something. Minor updates now only take a few seconds (like `arc liberate` and similar).
- Instead of dumping a sort-of-template file out, write out to a cache (`src/.cache/i18n_strings.json`). I'm planning to add more steps to read this cache and do interesting things with it (emit translatewiki strings…