store: Move PerAccountStoreWidget down into individual routes

[gnprice]

Fixes: #21

---

For combining this with #29, the diff looks like:

diff --git a/lib/widgets/content.dart b/lib/widgets/content.dart
index e970e4760..9998ab8d8 100644
--- a/lib/widgets/content.dart
+++ b/lib/widgets/content.dart
@@ -196,3 +196,4 @@ class MessageImage extends StatelessWidget {
       onTap: () {
-        Navigator.of(context).push(getLightboxRoute(message: message, src: resolvedSrc));
+        Navigator.of(context).push(getLightboxRoute(
+            context: context, message: message, src: resolvedSrc));
       },
diff --git a/lib/widgets/lightbox.dart b/lib/widgets/lightbox.dart
index bddbf6f38..c2d790591 100644
--- a/lib/widgets/lightbox.dart
+++ b/lib/widgets/lightbox.dart
@@ -7,2 +7,3 @@ import '../api/model/model.dart';
 import 'content.dart';
+import 'page.dart';
 import 'store.dart';
@@ -176,4 +177,9 @@ class _PageState extends State<_Page> {
 
-Route getLightboxRoute({required Message message, required String src}) {
-  return PageRouteBuilder(
+Route getLightboxRoute({
+  required BuildContext context,
+  required Message message,
+  required String src,
+}) {
+  return AccountPageRouteBuilder(
+    context: context,
     fullscreenDialog: true,

(This is also why this PR includes a PageRouteBuilder version of making a route, as well as a MaterialPageRoute version.)

[chrisbobbe]

Thanks, LGTM! Merging.