Settings: add a button to add remote EteSync accounts. (#768)

* Settings: add a button to add remote EteSync accounts.

EteSync is a fully open-source (clients and server), end-to-end encrypted,
and privacy respecting sync for your contacts, calendars and tasks.

This change was kept as close as possible to the existing CalDAV/DAVx5 button
so it made it possible for me to easily add translations for all languages that
already had the CalDAV button translated.

* Adding remote calendar: update wording as decided in #768.

As part of this I had to remove the translations as I don't speak those
languages and can't update them.