[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

https://github.com/twbs/bootstrap/issues/26512
https://github.com/twbs/bootstrap/issues/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 https://github.com/odoo/odoo/pull/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: 40dd121938

closes odoo/odoo#39541

closes odoo/odoo#39636

Original-signed-off-by: Aaron Bohy (aab) <aab@odoo.com>
X-original-commit: 7f72b837473f68a1dcd6a303ac1bf355dddcc2b1
Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>

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

@@ -160,7 +160,7 @@
                 <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>