MDL-60526 forms: Button elements should have type

The mform "buttons" should be of type "button", instead it's not specified so they get
the default of "submit". This is causing behat fails.

theme/boost/templates/core_form/element-button-inline.mustache

@@ -5,6 +5,7 @@
                 class="btn btn-secondary"
                 name="{{element.name}}"
                 id="{{element.id}}"
+                type="button"
                 {{#error}}
                     autofocus aria-describedby="id_error_{{element.name}}"
                 {{/error}}

theme/boost/templates/core_form/element-button.mustache

@@ -5,6 +5,7 @@
                 class="btn btn-secondary m-l-0"
                 name="{{element.name}}"
                 id="{{element.id}}"
+                type="button"
                 {{#error}}
                     autofocus aria-describedby="id_error_{{element.name}}"
                 {{/error}}