Create KnownAccount - a checkbox-like widget for remembering an account

Closes: #30 (closed)

This MR:

• Adds a new widget KnownAccount, which stores known accounts in profile data
• Adds a validation during Account object creation
• The above resulted in changing working_account to WorkingAccount | None (which was in TODO btw), because WorkingAccount("") is not valid anymore
• Make it possible to enter the dashboard with no working account (unset checkbox on SetAccount screen), previously it was always an working account
• small fixes of imports