LPS-34469 Suppor for adding pages from add panel --> PLEASE, READ DESCRIPTION FIRST :)

This pull has a lot of commits, but we have been very careful reviewing it.

The best way to review it is:

Compare all the commits together excluding: autogenerated (the last one) and the one with "no logic changes" (I promise there are no changes in that one)

After that, the only file which is hard to review is "add_page.jsp" because even though it contains some logic from edit_layout.jsp and init_attributes.jsp, this is quite different. We have reviewed this file many times though.

All the css and javascript has been reviewed by @ipeychev