feat(site): connect open_in parameter (#16036)

Second step to resolve [open_in
issue](https://github.com/coder/terraform-provider-coder/issues/297)

This PR improves the way the open_in parameter is forwarded across the
code, changing the last `string` to const everywhere.

Also make sure it is available and forwarded up to the `CreateLink`
component.

codersdk/agentsdk/convert.go

@@ -255,7 +255,6 @@ func AppFromProto(protoApp *proto.WorkspaceApp) (codersdk.WorkspaceApp, error) {
 		},
 		Health: health,
 		Hidden: protoApp.Hidden,
-		OpenIn: protoApp.OpenIn,
 	}, nil
 }
 
@@ -286,7 +285,6 @@ func ProtoFromApp(a codersdk.WorkspaceApp) (*proto.WorkspaceApp, error) {
 		},
 		Health: proto.WorkspaceApp_Health(health),
 		Hidden: a.Hidden,
-		OpenIn: a.OpenIn,
 	}, nil
 }