Fix #1562

[koskila]

Makes sure EnsurePagesLibrary ensures pages library for sites where "Site Pages" is not activated by activating the feature.

This is to address pnp/powershell#3959

[jansenbe]

@koskila : I would not just change the default behavior here as for example automatically enabling a SitePages library on a classic publishing portal is not the right thing to do in my mind. Can you change the PR like this:

1. Update the name of the private EnsurePagesLibraryAsync method to EnsurePagesLibraryImplementationAsync
2. Add a new EnsurePagesLibraryAsync method that calls the EnsurePagesLibraryImplementationAsync + adds your enhancement. This method can be made a public one by updating the IPages interface

This way the internal flow is not changed and we don't automatically create a SitePages library from existing code paths, if someone however wants this behavior then can call the public EnsurePagesLibraryAsync method