Add option to force E2EE encryption preference

Enabling this option ignores Autocrypt recommendation taking others
encryption preferences into account and overrides it with our own
encryption preference when possible.

This is similar to user always manually enabling/disabling encryption
manually in a classic Autocrypt-capable MUA UI whenever the control is
not disabled.

The goal is to allow encrypting responses to MUAs which can send
Autocrypt header but don't support setting encryption preference, such
as Thunderbird 91.

deltachat-ffi/deltachat.h

@@ -287,6 +287,8 @@ char*           dc_get_blobdir               (const dc_context_t* context);
  *                    To save traffic, however, the avatar is attached only as needed
  *                    and also recoded to a reasonable size.
  * - `e2ee_enabled` = 0=no end-to-end-encryption, 1=prefer end-to-end-encryption (default)
+ * - `e2ee_force`   = 1=ignore encryption preferences of others,
+ *                    0=use majority vote when deciding whether to encrypt (default).
  * - `mdns_enabled` = 0=do not send or request read receipts,
  *                    1=send and request read receipts (default)
  * - `bcc_self`     = 0=do not send a copy of outgoing messages to self (default),