Introduce a ContactId newtype

This makes the contact ID its own newtype instead of being a plain
u32.  The change purposefully does not yet try and reap any benefits
from this yet, instead aiming for a boring change that's easy to
review.  Only exception is the ToSql/FromSql as not doing that yet
would also have created churn in the database code and it is easier to
go straight for the right solution here.

src/test_utils.rs

@@ -851,7 +851,7 @@ async fn log_msg(context: &Context, prefix: impl AsRef<str>, msg: &Message) {
         &contact_name,
         contact_id,
         msgtext.unwrap_or_default(),
-        if msg.get_from_id() == 1u32 {
+        if msg.get_from_id() == DC_CONTACT_ID_SELF {
             ""
         } else if msg.get_state() == MessageState::InSeen {
             "[SEEN]"