fix issue #115

[lukasj25]

The extension currently uses a decomposition structure to extract local storage.
This indicates that it currently uses local to manage data.
For this extension to synchronize the settings with the Firefox account you must use "sync" instead of "local" storage.
For this, I had to replace references to local by « sync » in storage operations.

[GHolk]

I am not sure whether we need sync anything across devices,
or just sync the settings?

If we sync anything, we have to make sure that the history cache does not exceed the size limit,
and will the settings get overwriten if the history cache exceed the limit.

@wildskyf @soredake how do you think?

[soredake]

@GHolk syncing settings is enough in my opinion.