Enhancement: Consistent initial loading spinner

We have updated the loading spinner on the initial page load to run continuously during the client bootstrap. Previously, the spinner would appear and disappear multiple times.

Additionally, we have aligned the spinner's design with our other loading spinners and reduced the delay before it appears from 1 second to 0.5 seconds.

https://github.com/owncloud/web/pull/11054
https://github.com/owncloud/web/pull/11084
https://github.com/owncloud/web/issues/11041
https://github.com/owncloud/web/issues/11083
