Sign out

Add a Account Info section to Settings menu for displaying user name, space usage, space total size and sign out current account option.

Since space usage may change frequently, this PR used Service to fetch the lasted usage data from server.
Sign out should keep the account's email and server url, just delete its api token (e.g. set the `token` column to null in the database). This way, when next time the user wants to login this account again, he does need to type the server url and email.