[FIX] web: raw -> out of footer thing

Done this way so `_message` can be overridden with custom
markup. Should now work properly ootb: `final_message` should be a
`Markup` from qweb rendering, if `_message` also is such, the merge
should happen without further escaping of either value.

Note: needed to convert the str.__mod__ call to a concatenation as
otherwise the markupness information is lost. That's probably a
recurring issue.

addons/web/views/webclient_templates.xml

@@ -94,7 +94,7 @@
             </a>
         </t>
         <t t-set="final_message">Powered by %s%s</t>
-        <t t-raw="final_message % (odoo_logo, _message and ('- %s' % _message) or '')"/>
+        <t t-out="final_message % (odoo_logo, _message and ('- ' + _message) or '')"/>
     </template>
     <template id="brand_promotion" name="Brand Promotion">
         <div class="o_brand_promotion">