bug #19426 [Form] Fix the money form type render with Bootstrap3 (Th3Mouk)

This PR was merged into the 2.7 branch.

Discussion
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[Form] Fix the money form type render with Bootstrap3

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | Part fixing https://github.com/symfony/symfony/issues/19424
| License       | MIT
| Doc PR        | none

There is a confusion between the variable naming, and the result expected.

When prepend variable is false, the currency symbol must be add after the widget.
When the `money_pattern`starts with `{{`, `prepend` variable must be `false`.

Commits
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637a441 Fix the money form type render with Bootstrap3

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

@@ -21,12 +21,12 @@
 
 {% block money_widget -%}
     <div class="input-group">
-        {% set prepend = '{{' == money_pattern[0:2] %}
+        {% set append = money_pattern starts with '{{' %}
-        {% if not prepend %}
+        {% if not append %}
             <span class="input-group-addon">{{ money_pattern|replace({ '{{ widget }}':''}) }}</span>
         {% endif %}
         {{- block('form_widget_simple') -}}
-        {% if prepend %}
+        {% if append %}
             <span class="input-group-addon">{{ money_pattern|replace({ '{{ widget }}':''}) }}</span>
         {% endif %}
     </div>