bug #13883 #13857 Added default button class (Piers Warmers)

This PR was submitted for the 2.7 branch but it was merged into the 2.6 branch instead (closes #13883).

Discussion
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#13857 Added default button class

[Bridge\Twig] Added default-btn class to Twig button block.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | n/a
| Fixed tickets | #13857
| License       | MIT
| Doc PR        | n/a

Commits
-------

c07a902 Added default button class

src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_3_layout.html.twig

@@ -15,7 +15,7 @@
 {%- endblock textarea_widget %}
 
 {% block button_widget -%}
-    {% set attr = attr|merge({class: (attr.class|default('') ~ ' btn')|trim}) %}
+    {% set attr = attr|merge({class: (attr.class|default('btn-default') ~ ' btn')|trim}) %}
     {{- parent() -}}
 {%- endblock %}