reintroduce All Accounts view and set as default

As an intermediate step to a unified inbox, we should get rid of the account switcher.

Left to do:
* [ ] put in headers for the »Inbox, Sent, Trash« etc folder groups. The email address of the respective account should be shown above each set. @DeepDiver1975 can you help how to get that to the template? I tried a few things but didn’t succeed.
* [ ] finish #233 (possibility to delete account) by @Gomez – so there is an »Add account« function in the settings
* [ ] remove the account switcher dropdown

In the future we should also by default collapse the sets of folders for each account so that it doesn’t take over the whole sidebar.