[FIX] website: BS4, smaller btn-sm buttons

Before BS4 migration, Odoo was not very consistent and used mainly
btn-sm buttons and btn-xs buttons. We took advantage of the fact
BS4 removed the btn-xs class to convert our btn-sm to standard btn
and our btn-xs to standard btn-sm.

While this is perfect in theory, the new btn-sm class does not
generated small enough design for us. This commit changes the BS4
standard style for our website, making btn-sm smaller.

addons/website/static/src/scss/bootstrap_overridden.scss

@@ -8,4 +8,8 @@ $headings-color: color('h1');
 $font-family-sans-serif: nth($o-theme-fonts, $o-theme-font-number);
 $headings-font-family: nth($o-theme-fonts, $o-theme-headings-font-number);
 
-$input-btn-border-width: 2px;
+$input-btn-border-width: 2px;
+
+$font-size-sm: 0.75rem;
+$btn-padding-y-sm: 0.0625rem;
+$btn-padding-x-sm: 0.3125rem;