Google Reader starred.json importer

<del>Hey there, wrote a somewhat hacked together piece of code to import starred items from a starred.json file. Worked for me on an initial bit of testing, though it'll ignore stars on feeds that aren't subscribed to.

<del>How it works currently is that I've added a new page at `/stars/import`, which'll go through items (that are part of currently subscribed feeds), adds them if they don't exist (code needs cleaning up), and then set them to read + starred.

Definitely not expecting to get it merged in its current state, but I'd like to know whether this functionality is a good fit and if so what's the best way to integrate it.

- <del>[x] Clean up controller code</del>
- [x] Clean up import command code (?)
- <del>[ ] Better copy on import page</del>
- <del>[ ] Use localisation instead of hardcoded string</del>
- [x] Add tests
- [x] Fix ignoring stars on non-subscribed feeds (ignore/warn/auto-subscribe/ask?)