[FIX] website, *: correct snippet addition option

* website_mass_mailing, website_twitter

A snippet option describes where standard snippets can be dropped in the
page (which is currently in oe_structure elements or html fields, except
for the footer which can only have one snippet). Some external snippets
in other apps or themes were not using this snippet option but defined
their owns which were not adapted to the correct drop locations.

addons/website/views/snippets.xml

@@ -1277,7 +1277,8 @@
         data-exclude=".s_col_no_resize > [class*='col-lg-']">
     </div>
 
-    <div data-selector="section, :not(.o_gallery > .container) > .carousel, .parallax"
+    <div id="so_snippet_addition"
+        data-selector="section, :not(.o_gallery > .container) > .carousel, .parallax"
         data-drop-in=":not(p).oe_structure:not(.oe_structure_solo), :not(p)[data-oe-type=html], :not(p).oe_structure.oe_structure_solo:not(:has(> section, > div))"/>
 
     <div data-js="content"

addons/website_mass_mailing/views/snippets_templates.xml

@@ -51,6 +51,9 @@
 </template>
 
 <template id="newsletter_subscribe_options" name="Newsletter Subscribe Options" inherit_id="website.snippet_options">
+    <xpath expr="//*[@id='so_snippet_addition']" position="attributes">
+        <attribute name="data-selector" add=".o_newsletter_popup" separator=","/>
+    </xpath>
     <xpath expr="//div" position="after">
         <div data-js="mailing_list_subscribe"
             data-selector=".js_subscribe"
@@ -58,8 +61,7 @@
             <a tabindex="-1" href="#" class="dropdown-item" data-select_mailing_list="" data-no-preview="true">Change Newsletter</a>
         </div>
         <div data-js="newsletter_popup"
-            data-selector=".o_newsletter_popup"
-            data-drop-in=".oe_structure, [data-oe-type=html]">
+            data-selector=".o_newsletter_popup">
             <a tabindex="-1" href="#" class="dropdown-item" data-select_mailing_list="" data-no-preview="true">Change Newsletter</a>
         </div>
     </xpath>

addons/website_twitter/views/website_twitter_snippet_templates.xml

@@ -17,9 +17,7 @@
 
     <template id="website_twitter_options" inherit_id="website.snippet_options">
         <xpath expr="." position="inside">
-            <div data-js='twitter'
-                data-selector=".twitter"
-                data-drop-in=".oe_structure, [data-oe-type=html]"/>
+            <div data-js="twitter" data-selector=".twitter"/>
         </xpath>
     </template>