ability to print cards & weigh-in forms for multiple sessions

[jflamy]

From the session screen, ability to either multiselect or filter sessions in order to produce athlete cards + challenge cards

See https://cookbook.vaadin.com/grid-conditional-select . Contrary to the active age groups page, we don't want being selected to update the session. We want to collect the selected sessions.

There is also an add-on to add shift-click selection, but not on the checkboxes. Not sure this is intuitive (or whether the checkbox selection can be added). https://vaadin.com/directory/component/selection-grid