fix(ui): Fix custom outgoing stamp and fix stamp command help

Replace the hardcoded "me" stamp for outgoing messages that are
sometimes used with the value from PREF_OUTGOING_STAMP.

It was never implemented in all code paths. Like with receipts for
example.

Additionally fix the confusing description and variable names.

Ref: 2c003dd2 (Add /stamp feature)
Ref: 3a4cd7da (Broke the variables)
Fixes: https://github.com/profanity-im/profanity/issues/2125
Signed-off-by: Michael Vetter <jubalh@iodoru.org>

src/xmpp/message.c

@@ -76,7 +76,7 @@ gchar*
 get_display_name(const ProfMessage* const message, int* flags)
 {
     if (equals_our_barejid(message->from_jid->barejid)) {
-        return g_strdup("me");
+        return prefs_get_string(PREF_OUTGOING_STAMP);
     } else {
         if (flags)
             *flags = NO_ME;