[FIX] project_issue: use issue_count instead of counting issues

It is costly to compute all issues, return the full list of all issues, only to
compute the length and display it.
Use proper issue_count field.

Closes #18858

addons/project_issue/views/project_project_views.xml

@@ -24,7 +24,7 @@
         <field name="arch" type="xml">
             <field name="use_tasks" position="after">
                 <field name="use_issues"/>
-                <field name="issue_ids" invisible="1"/>
+                <field name="issue_count" invisible="1"/>
                 <field name="issue_needaction_count" invisible="1"/>
             </field>
             <xpath expr="//div[contains(@class, 'o_kanban_manage_reports')]/div[last()]" position="after">
@@ -35,7 +35,7 @@
             <xpath expr="//div[contains(@class, 'o_project_kanban_box')]/div[last()]" position="before">
                 <div attrs="{'invisible': [('use_issues', '=', False)]}" class="o_project_kanban_box">
                     <a name="%(act_project_project_2_project_issue_all)d" type="action">
-                        <span class="o_value"><t t-raw="record.issue_ids.raw_value.length"/></span>
+                        <span class="o_value"><t t-raw="record.issue_count.raw_value"/></span>
                         <span class="o_label"><field name="label_issues"/></span>
                         <a t-if="record.issue_needaction_count.raw_value" class="o_needaction" name="%(act_project_project_2_project_issue_all)d" type="action" context="{'search_default_message_needaction': 1}">
                             <t t-esc="record.issue_needaction_count.raw_value"/>