add forgotten template for button opening the multiselect modal

Former-commit-id: d5b84e56853b0ceb4e2369bf33a033ccaacefcff

ippisite/ippidb/templates/multiselection_button.html

+<button type="button" class="btn btn-primary btn-lg m-1" style="width: inherit!important" data-toggle="modal" data-target="#modal-{{ param_name }}">
+  <i class="fa fa-filter"></i>{{ label }}
+</button>