Bugfix: Fix URL resolution for app provider

We fixed a bug in the URL resolution for the app provider.
If owncloud is installed to a non-root directory we computed a wrong URL for the app provider urls.
This could trigger a connection validation in the client, which intern triggered a request to the app provider.
Resulting in an endless loop rendering the client unusable.

https://github.com/owncloud/client/issues/11296
