[FIX] web: fix misaligned select inputs

Browsers add a 1px padding to inputs but they don't do it for the
select elements, which makes them unaligned.

Tested under Chrome 68, Firefox 61 and Safari 11 on MacOS 10.13.6.

addons/web/static/src/scss/fields.scss

@@ -32,6 +32,11 @@
         text-align: inherit;
     }
 
+    // Selection fields
+    &, select.o_input {
+        padding: 1px;  // Other inputs get a 1px padding automatically added by the browsers but selects do not
+    }
+
     // Block fields
     &.note-editor, &.oe_form_field_html, &.oe_form_field_html_text, &.oe_form_field_text, &.o_field_domain, &.o_graph_linechart, &.o_graph_barchart {
         display: block;