[FIX] web: correctly render "font awesome" icons on buttons

Revision on https://github.com/odoo/odoo/commit/9de1bc0

From the commit above, the style of some field binary images are broken.
This is due to changes from `<span>` to `<button>`. The style has been
altered by the default browser's style, which does the following on
Google Chrome:

```
    border-color: white;
    border: 1px solid;
    padding: 1px 7px 2px;
```

Most browsers set a border and some padding on buttons. This commit fixes
the style specifically for "font awesome" icons on buttons, by removing
the border and by overwriting the padding.

Tested browsers:

  - Google Chrome 67
  - Mozilla Firefox 61
  - Safari 11

Task-ID 1864027

Closes #25691

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

@@ -355,6 +355,11 @@
             opacity: 0;
             transition: opacity ease 400ms;
 
+            > button.fa {
+                border: none;
+                background-color: transparent;
+            }
+
             > .fa {
                 margin: 5px;
                 cursor: pointer;