Wrap each addon settings form in their own form tag

- The single form tag was preventing a given addon settings to be saved if another addon had an empty required field.
- Instead of concatenating the addon form HTML through Hook::callAll, we loop manually through the hooks, appending the HTML to an array.

view/templates/settings/addons.tpl

@@ -1,11 +1,14 @@
-
 <h1>{{$title}}</h1>
 
+{{foreach $addon_settings_forms as $addon_settings_form}}
 
 <form action="settings/addon" method="post" autocomplete="off">
-<input type='hidden' name='form_security_token' value='{{$form_security_token}}'>
-
-{{$settings_addons nofilter}}
-
+	<input type="hidden" name="form_security_token" value="{{$form_security_token}}">
+	{{$addon_settings_form nofilter}}
 </form>
 
+{{foreachelse}}
+
+<p>{{$no_addon_settings_configured}}</p>
+
+{{/foreach}}