JMAP: Implement listing sub-accounts

[benbucksch]

Implement listPossibleSubAccounts() and mayHaveSubAccounts

• It must list all addressbooks and calendars (other than those which are already dependent accounts)
• It must not list other than special generated calendars/addressbooks like "birthdays" or "last accessed"
• It should not list delegated accounts that the user has access to.

[benbucksch]

UI: Settings | Mail | (account) | General panel | Sub-Accounts | Button (+) on the right of the header