Keep WebPush settings

[abcang]

When logging out, the WebPush setting disappears. So, I propose a feature to save the WebPush setting to localStorage and restore it at login.

[ykzts]

cc @sorin-davidoi

[sorin-davidoi]

Code-wise, I would also remove the setting when logging out. When Safari implements Service Worker and Push Notifications, we might run into this, though we should probably handle it then.

From a technical perspective, I generally don't like this kind of automatic migration of settings between sessions - it is either a global setting or a session setting. There is also a risk of the settings persisting between users (if the session expires in the backend and another users logs in, they will inherit the settings of the previous user). I don't opose it, but I think we should consult more people to see if this kind of behavior is expected.

[abcang]

I want to solve this problem because it seems like a bug that WebPush settings disappear when logging out. So, I think that another method that does not use localStorage is acceptable (for example, setting is saved for each account, not for each session). I will close this PR if there are other good solutions.

Also, in this PR, since the setting is saved with the key containing the account id in localStorage, the settings of others are not applied at login.