Parse only campsite type and dates when getting booking form

If i parse everything, then the campsite type options no longer have
their minimum values preset, meaning that people have to input each and
every one of them.

pkg/booking/public.go

@@ -38,7 +38,10 @@ func (h *PublicHandler) Handler(user *auth.User, company *auth.Company, conn *da
 			switch r.Method {
 			case http.MethodGet:
 				page := newPublicPage(r.Context(), company, conn, user.Locale)
-				_ = page.Form.Parse(r) // Get query parameters, if any
+				_ = r.ParseForm()
+				page.Form.CampsiteType.FillValue(r)
+				page.Form.ArrivalDate.FillValue(r)
+				page.Form.DepartureDate.FillValue(r)
 				page.MustRender(w, r, user, company, conn)
 			case http.MethodPost:
 				makeReservation(w, r, user, company, conn)