[FIX] web: dropdowns can't be in a relative-positioned element

Before this commit, when a dropdown overflew its container
i.e. in the case of a long filter menu in modal
The scrolling of that dropdown to get to Add custom Filter
was impossible

This was because dropdowns react pretty bad when contained in a
relative positioned container

twbs/bootstrap#26512
twbs/bootstrap#28513 !!

After this commit, the btn-group that adds the relative positioning
is forced into the default value

This commit corrects what was initially
corrected at odoo#37594
in v12.0.
The incriminating commit that retriggers the issue
is irrelevant because it is the refactoring of action manager
but here it is: 40dd121

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

@@ -195,7 +195,7 @@
             <aside class="o_cp_sidebar"/>
         </div>
         <div class="o_cp_right">
-            <div class="btn-group o_search_options" role="search"/>
+            <div class="btn-group o_search_options position-static" role="search"/>
             <nav class="o_cp_pager" role="search" aria-label="Pager"/>
             <nav class="btn-group o_cp_switch_buttons" role="toolbar" aria-label="View switcher"/>
         </div>