[FIX] web: dropdowns can't be in a relative-positioned element
[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: 40dd121938552d3f88289d2e5fcd1d57a0dd01bb

closes odoo/odoo#39541

closes odoo/odoo#39636

Original-signed-off-by: Aaron Bohy (aab) <40b904fd8852297daeaeb426b1bca46fd2454aa3@odoo.com>
X-original-commit: 7f72b837473f68a1dcd6a303ac1bf355dddcc2b1
Signed-off-by: Lucas Perais (lpe) <42d29efea96f3992aa24782120ec2948e42548fa@odoo.com>

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