[FIX] web_editor: secure the paddings of btn in the editor

Prevent the padding of editor .btn's to change when a theme modifies
them (by mistake or not).

Once again, this is a problem of css isolation between the editor and
the website customization. A theme cannot safely, at this time, do
"#wrapwrap .btn" to isolate its rules because it may override some
bootstrap rules like ".btn-group .btn" which would became less
important. It cannot changes the bootstrap variables either as it is
still used by the odoo ui in some places.
There is no safe good solution to "share" bootstrap theming between
odoo ui and website ui. The safest solution is to force the design
of odoo ui elements that may be overridden by themes (in the end,
it's the same as progressively removing bootstrap from odoo ui).

addons/web_editor/static/src/less/web_editor.ui.less

@@ -665,6 +665,7 @@ html[lang] > body.editor_enable{
             height: @odoo-navbar-height;
             border: none;
             border-radius: 0;
+            padding: @padding-base-vertical @padding-base-horizontal; // force this because of themes
             background-color: @o-we-color-dark;
             color: @o-we-color-text-normal;
             border-top: 2px solid transparent;