consider using indexed db instead of localstorage

[ichub]

No description provided.

[robknight]

IndexedDB has a much larger maximum size than localstorage, so we should check that we have not made assumptions about the maximum size of the local DB for the purposes of sync/export which would be broken by this change, e.g. max HTTP upload sizes. The current export mechanism relies on data URLs, which have a maximum size of 20MB - larger than the local storage max, but smaller than would be possible with indexeddb.