Modern attachments (#505)

* Minimum API version is now 100

* Remove old API features

- Anything below API v100 no longer supported

* Reefactor attachment widget to support modern attachment API

* Filter and display attachments correctly

* Refactor

test/api_test.dart

@@ -110,10 +110,6 @@ void main() {
 
       // Check supported functions
       assert(api.apiVersion >= 50);
-      assert(api.supportsSettings);
-      assert(api.supportsNotifications);
-      assert(api.supportsPoReceive);
-
       assert(api.serverInstance.isNotEmpty);
       assert(api.serverVersion.isNotEmpty);