Multi Account Labels and more

I suggest a few closely related UI updates to improve usability for users with multiple accounts.

  1. Give users a way to create/edit labels for their own accounts (e.g. “work”, “friends”, “family”)

  2. On the smartphone app show the label of the active account at the top of the screen instead of the text “Delta Chat”

  3. Have a setting to control which screen Delta Chat displays when the app is first opened after starting the device: either the chats of the account which was active the last time Delta Chat was used (current behavior) or the account switcher screen