Properly handle changes to the data format in the local storage

[mtneug]

Unexpected changes to the data format in the local storage can lead to non-working site (see #1381 and #1395). There should be one agreed-upon way to migrate to newer versions without adding guards to each local storage access.

[Arnei]

Since we are using redux-persist, we should probably leverage its versioning ability which allows us to provide a migration function. See also https://github.com/rt2zz/redux-persist/blob/master/docs/migrations.md