apache: Add frontpage shortcut for showing user dir functionality

Currently, by default FreedomBox has userdir dir functionality where public_html directory in each user's home directory exposed as /~username/ on FreedomBox. This is not shown to users anywhere. Create a shortcut in the frontpage to have a link to user's public directory.

We could create one shortcut for each user in the system. But this will expose the list of users to the outside world. Instead only expose each logged-in user's public directory to themselves.