Datatables: Remember report configuration even when not displayed in dashboard

[tsteur]

Feedback from user re Pages:
This site is pretty hard to read, as my site structure looks typically like "example.com/123/post-title", thus it'll only show the post id as the URL and the "sub-URL", here the post title, only when I expand it. In my opinion this is over-structured. Even on a huge site it might still be easier to read when there is no URL grouping at all.

For this user the "flatten" feature makes sense I think. Similar to the dashboard we should remember which visualization, which limit, which parameters (eg flatten, expanded) a user has selected and apply them next time again. Otherwise one always has to scroll down to the end of the report and switch to flatten

[mattab]

See #1915