name special contact-ids where easily possible

the point of this pr is to get an overview
how and where DC_CONTACT_ID_DEVICE is used,
to prepare introducing a device-"chat".

i did not change the sql statements for now
as this would require some more refactoring
and has the potential to introduce bugs.

src/contact.rs

@@ -264,7 +264,7 @@ impl Contact {
             .unwrap_or_default();
 
         if addr_normalized == addr_self {
-            return 1;
+            return DC_CONTACT_ID_SELF;
         }
 
         context.sql.query_get_value(
@@ -301,7 +301,7 @@ impl Contact {
             .unwrap_or_default();
 
         if addr == addr_self {
-            return Ok((1, sth_modified));
+            return Ok((DC_CONTACT_ID_SELF, sth_modified));
         }
 
         if !may_be_valid_addr(&addr) {