Move key loading from deprecated Key struct to DcKey trait

This moves the loading of the keys from the database to the trait and
thus with types differing between public and secret keys.  This
fetches the Config::ConfiguredAddr (configured_addr) directly from the
database in the SQL to simplify the API and consistency instead of
making this the responsiblity of all callers to get this right.

Since anyone invoking these methods also wants to be sure the keys
exist, move key generation here as well.  This already simplifies some
code in contact.rs and will eventually replace all manual checks for
existing keys.

To make errors more manageable this gives EmailAddress it's own error
type and adds some conversions for it.  Otherwise the general error
type leaks to far.  The EmailAddress type also gets its ToSql trait impl
to be able to save it to the database directly.

src/securejoin.rs

@@ -12,7 +12,7 @@ use crate::e2ee::*;
 use crate::error::{bail, Error};
 use crate::events::Event;
 use crate::headerdef::HeaderDef;
-use crate::key::{dc_normalize_fingerprint, Key};
+use crate::key::{dc_normalize_fingerprint, DcKey, Key, SignedPublicKey};
 use crate::lot::LotState;
 use crate::message::Message;
 use crate::mimeparser::*;
@@ -135,12 +135,13 @@ pub fn dc_get_securejoin_qr(context: &Context, group_chat_id: ChatId) -> Option<
 }
 
 fn get_self_fingerprint(context: &Context) -> Option<String> {
-    if let Some(self_addr) = context.get_config(Config::ConfiguredAddr) {
-        if let Some(key) = Key::from_self_public(context, self_addr, &context.sql) {
-            return Some(key.fingerprint());
+    match SignedPublicKey::load_self(context) {
+        Ok(key) => Some(Key::from(key).fingerprint()),
+        Err(_) => {
+            warn!(context, "get_self_fingerprint(): failed to load key");
+            None
         }
     }
-    None
 }
 
 /// Take a scanned QR-code and do the setup-contact/join-group handshake.