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.
because other database like `certs.db`, `photo.db`, `data.db`, `avatar.db`, all of them using lower letters except `Account.db`. So I want to keep them in a uniform way.