Add hidden MAM setting and trigger MAM retrievel when opening new window

Only when we start the conversation.
Not yet when we get messaged and a new window is opened.
Need to have sorting of messages in the window buffer then, I guess.
Also MAM IQ should only be send one time in such a case.

If MAM is enabled history from sql backend will not be shown.

`mam` in profrc enables experimental MAM.
Can change soon again. Don't rely on stuff in this stage ;)

src/config/preferences.h

@@ -170,6 +170,7 @@ typedef enum {
     PREF_HISTORY_COLOR_MUC,
     PREF_AVATAR_CMD,
     PREF_SLASH_GUARD,
+    PREF_MAM,
 } preference_t;
 
 typedef struct prof_alias_t {