api!: Contact::get_color(): Preserve address-based color hue for SELF

4010c60e7b "feat: use key fingerprints for color generation" changes
colors for contacts including SELF. Even if an avatar is set, the self-color is visible in
e.g. replies to outgoing messages.

This adds `Config::Selfcolor` and sets it in a migration. This doesn't preserve the old
address-based color accurately because the old code generating colors is already dropped, so this
only preserves the color angle (hue).

deltachat-ffi/src/lib.rs

@@ -4311,7 +4311,8 @@ pub unsafe extern "C" fn dc_contact_get_color(contact: *mut dc_contact_t) -> u32
         return 0;
     }
     let ffi_contact = &*contact;
-    ffi_contact.contact.get_color()
+    let ctx = &*ffi_contact.context;
+    block_on(ffi_contact.contact.get_color(ctx)).unwrap_or_log_default(ctx, "Failed get_color")
 }
 
 #[no_mangle]