[FIX] web: set title on Many2Many tags

When the content of a Many2Many tag is too large, it is truncated. This
is an issue since there is no easy way to see it.

The fix adds a title on the tag with the content.

X-port of commit odoo/enterprise@bd3bfe0422

addons/web/static/src/xml/base.xml

@@ -750,7 +750,7 @@
 <t t-name="FieldMany2ManyTag">
     <t t-foreach="elements" t-as="el">
         <span t-attf-class="badge dropdown #{'o_tag_color_' + (el['color'] || 0)}" t-att-data-color="(el['color'] || 0)" t-att-data-index="el_index" t-att-data-id="el['id']">
-            <span class="o_badge_text"><t t-raw="el['display_name']"/></span>
+            <span class="o_badge_text" t-attf-title="#{el['display_name']}"><t t-raw="el['display_name']"/></span>
             <span t-if="!readonly" class="fa fa-times o_delete"/>
         </span>
     </t>