Make /sendfile in OMEMO session configurable

`/omemo sendfile on` allows unencrypted file transfer in an OMEMO
session.

Regards https://github.com/profanity-im/profanity/pull/1270

src/config/preferences.h

@@ -160,6 +160,7 @@ typedef enum {
     PREF_STATUSBAR_ROOM,
     PREF_OMEMO_LOG,
     PREF_OMEMO_POLICY,
+    PREF_OMEMO_SENDFILE,
     PREF_OCCUPANTS_WRAP,
     PREF_CORRECTION_ALLOW,
 } preference_t;