Add OMEMO policy

There is 3 policy:

- manual: OMEMO session are only started manually
- automatic: OMEMO session are only started if they have been started
  manually before
- always: OMEMO session are always started unless they have been ended
  manually before

Closes #1040 and fixes #1052

src/config/preferences.h

@@ -149,6 +149,7 @@ typedef enum {
     PREF_STATUSBAR_CHAT,
     PREF_STATUSBAR_ROOM,
     PREF_OMEMO_LOG,
+    PREF_OMEMO_POLICY,
 } preference_t;
 
 typedef struct prof_alias_t {