Our caching of emoji in localStorage is tricky and barely tested. This
adds the following test cases:

- when the localStorage is empty
- when the localStorage stores the correct version
- when the localStorage stores an incorrect version

These cases are handled _fine_ currently. The following cases are not
handled well today:

1. when the localStorage contains garbage data (e.g. it got corrupted or
   manipulated somehow). We currently do not catch the error.
2. when the localStorage cannot save the item. We thought we handled the
   case correctly, _but_: We first try to save the `EMOJI_VERSION` in
   localStorage _and_ then the big blob. So if the saving fails, we have
   updated the version but not the blob.