Cannot reset local storage if local DB cannot be opened

[robertknight]

In the web app and Firefox extension, if the local DB cannot be opened for any reason then the user cannot reset the app back to a working state easily.

The app menu has a 'Clear Offline Cache' option but this is only accessible when the store is unlocked.

This issue has bitten me a few times when switching from a Nightly version of Firefox to a stable build which uses an older and incompatible IndexedDB format.

In the Chrome extension, removing and re-installing the extension fixes the issue as it wipes local extension data. The same doesn't happen in Firefox though. In the web app, browsers do provide a way to clear offline storage but this is typically well hidden in the browser.