realm [nfc]: Add HasRealmStore for other substores to use

The other substores could always handle this themselves individually,
like Unreads does today with ChannelStore.  But this is one substore
that lots of others are going to want references to, so we might as
well make that a bit easier.

Author: gnprice

lib/model/realm.dart

@@ -10,7 +10,8 @@ import 'store.dart';
 ///
 /// See also:
 ///  * [RealmStoreImpl] for the implementation of this that does the work.
-mixin RealmStore {
+///  * [HasRealmStore] for an implementation useful for other substores.
+mixin RealmStore on PerAccountStoreBase {
   int get serverPresencePingIntervalSeconds;
   int get serverPresenceOfflineThresholdSeconds;
 
@@ -68,6 +69,17 @@ mixin ProxyRealmStore on RealmStore {
   EmailAddressVisibility? get emailAddressVisibility => realmStore.emailAddressVisibility;
 }
 
+/// A base class for [PerAccountStore] substores that need access to [RealmStore]
+/// as well as to [CorePerAccountStore].
+abstract class HasRealmStore extends PerAccountStoreBase with RealmStore, ProxyRealmStore {
+  HasRealmStore({required RealmStore realm})
+    : realmStore = realm, super(core: realm.core);
+
+  @protected
+  @override
+  final RealmStore realmStore;
+}
+
 /// The implementation of [RealmStore] that does the work.
 class RealmStoreImpl extends PerAccountStoreBase with RealmStore {
   RealmStoreImpl({