SAK-47706 Extend Add Items From Another Page to include pages from other sites

[jkozar2]

This PR adds a site picker in Lessons' Add Items From Another Page function. When the user chooses a site, pages from that site will be made available and the user can import certain items from those pages. They will then be reorderable and individually selectable on the following page, and subsequently imported into the currently edited page, just like items added from another page in the same site. https://sakaiproject.atlassian.net/browse/SAK-47706