[FIX] web: clear all BS4 alerts

Commit https://github.com/odoo/odoo/commit/8caf84f70ada259f041b424d37686845604d182a
was not handling the case where there was an invisible DOM element
between the button box and the alert component. Let us clear all alerts.

This commit also introduces a way to properly fix/extend bootstrap in
future fixes/updates (some of our rules may already be moved in that
file once it is forward-ported to master).

closes odoo/odoo#27662

addons/web/static/src/scss/bootstrap_review.scss

@@ -0,0 +1,10 @@
+///
+/// This file regroups the CSS rules made to fix/extend bootstrap in all places
+/// where it is used in Odoo (backend / frontend / reports / ...)
+///
+
+.alert {
+    // Alerts are block elements with relative positioning.
+    // They would go over floating elements, which is never what we want.
+    clear: both;
+}

addons/web/static/src/scss/form_view_extra.scss

@@ -41,6 +41,8 @@
             }
         }
 
+        // TODO remove me in master (this rule is a copy of the one in the
+        // bootstrap_review.scss file)
         + .alert {
             clear: both;
         }

addons/web/views/webclient_templates.xml

@@ -40,6 +40,7 @@
         <link rel="stylesheet" type="text/scss" href="/web/static/src/scss/import_bootstrap_part1.scss"/>
         <link rel="stylesheet" type="text/scss" href="/web/static/src/scss/bs_mixins_overrides.scss"/>
         <link rel="stylesheet" type="text/scss" href="/web/static/src/scss/import_bootstrap_part2.scss"/>
+        <link rel="stylesheet" type="text/scss" href="/web/static/src/scss/bootstrap_review.scss"/>
     </template>
 
     <template id="assets_common" name="Common Assets (used in backend interface and website)">