Make /sendfile in OTR session configurable

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

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

src/config/preferences.c

@@ -1790,6 +1790,7 @@ _get_group(preference_t pref)
             return PREF_GROUP_CONNECTION;
         case PREF_OTR_LOG:
         case PREF_OTR_POLICY:
+        case PREF_OTR_SENDFILE:
             return PREF_GROUP_OTR;
         case PREF_PGP_LOG:
             return PREF_GROUP_PGP;
@@ -1916,6 +1917,8 @@ _get_key(preference_t pref)
             return "log";
         case PREF_OTR_POLICY:
             return "policy";
+        case PREF_OTR_SENDFILE:
+            return "sendfile";
         case PREF_LOG_ROTATE:
             return "rotate";
         case PREF_LOG_SHARED:

src/config/preferences.h

@@ -135,6 +135,7 @@ typedef enum {
     PREF_LOG_SHARED,
     PREF_OTR_LOG,
     PREF_OTR_POLICY,
+    PREF_OTR_SENDFILE,
     PREF_RESOURCE_TITLE,
     PREF_RESOURCE_MESSAGE,
     PREF_INPBLOCK_DYNAMIC,