"Open in Web" skips app query param for web

## Describe the bug
While debugging https://github.com/owncloud/web/issues/11108 I found out that the URL that the backend creates doesn't contain an `app` query param. While I mitigated that in web (see https://github.com/owncloud/web/pull/11113 ) the URL should still be fixed backend side.

Also, looks like this line should be removed meanwhile: https://github.com/owncloud/ocis/blob/eb5a9ceade9e8304d53231478b59aa50f74368b7/services/frontend/pkg/revaconfig/config.go#L137 (needs to be tested though... the linked PR is merged)

## Steps to reproduce
1. Create a `.docx` file locally and wait until it's synced to the server via desktop client
2. Use `Open in OnlyOffice` from your local file explorer context menu

## Expected behavior
The URL that's generated server side contains a query parameter `app=OnlyOffice`

## Actual behavior
The query parameter is missing