Issue #3611920: Reword the seat-map page title to 'Choose your seats' since several seats can be selected

The graphical seat map lets a visitor pick several seats, but the page title read "Choose a seat: [event]", which implies a single seat.

This rewords it to "Choose your seats: [event]" (the page heading, the browser title and the accessible page title), matching the "Your selection" panel. The French translation becomes "Choisir vos places : [event]".

Follow-up to the graphical seat map (#3611792).