Add /occupants indent command

Roster has a `/roster contact indent` option.
Now we have the same for occupants. So contacts in roster and in MUC can
have configurable indentation.

Regards https://github.com/boothj5/profanity/issues/690

src/config/preferences.h

@@ -246,6 +246,8 @@ gint prefs_get_roster_resource_indent(void);
 void prefs_set_roster_resource_indent(gint value);
 gint prefs_get_roster_presence_indent(void);
 void prefs_set_roster_presence_indent(gint value);
+gint prefs_get_occupants_indent(void);
+void prefs_set_occupants_indent(gint value);
 
 void prefs_add_login(const char *jid);