Save agGrid's column visibility and order in session obj

agGrid provides access to both, the column state and the sort model (the visibility and position of a column respectively) via their gridOptions API. As @mpdehaan suggested, we should save those user preferences to improve UX. Here is a [StackOverflow ticket](https://stackoverflow.com/questions/42811020/how-do-i-get-and-set-ag-grid-state) for quick reference.