[FIX] web: dropdown boundary overflow

Open a search view modal ("search more" on a m2o)
Select a filter that will make records fall to zero
The modal will shrink
Trigger the opening of a dropdown (Filters), and make sure
that there are many many menu items there (many filters)

Before this commit, the dropdown was bigger than the modal
which had a scroll bar,
however, the dropdown itself was not scrollable properly
So, clicking in it to select a menu item will fail when scrolled
all the way down, because elements did not actually followed the scroll

After this commit, the dropdown follows the scroll, and it is possible
to use it even way down

We use a behavior of bootstrap 4 itself:
FIX: twbs/bootstrap#24976
ISSUE: twbs/bootstrap#24251

OPW 2076337

X-original-commit: c3273b7

addons/web/static/src/xml/base.xml

@@ -1159,7 +1159,7 @@
 </t>
 
 <div t-name="web.DropdownMenu" t-att-class="widget.dropdownStyle.el.class" t-att="widget.dropdownStyle.el.attrs">
-    <button t-att-class="widget.dropdownStyle.mainButton.class" data-toggle="dropdown" aria-expanded="false" tabindex="-1">
+    <button t-att-class="widget.dropdownStyle.mainButton.class" data-toggle="dropdown" aria-expanded="false" tabindex="-1" data-boundary="viewport">
         <span t-att-class="widget.dropdownIcon"/> <t t-esc="widget.dropdownTitle"/> <span t-if="widget.dropdownSymbol" t-att-class="widget.dropdownSymbol"/>
     </button>
     <div class="dropdown-menu o_dropdown_menu" role="menu">