[FIX] web: correct style on btn-link class

Due to the switch from less to scss, changes were made. Especially, the
class oe_link is now 'deprecated' and interpreted as 'btn-link'. The
previous style defined on the latter removed the padding which breaks
the style when several buttons follow each other.

This commit commit should fix the style while maintaining the intended
behaviour, a vertical alignement between the button and the text inside
the pane.

addons/web/static/src/scss/form_view.scss

@@ -733,8 +733,8 @@ label.o_light_label, .o_light_label label {
             .o_field_widget {
                 width: 50%;
             }
-            button.btn-link {
-                padding: 0;
+            button.btn-link:first-child {
+                padding-left: 0;
             }
             a.oe-link {
                 font-size: 12px;