[IMP] sale_timesheet,_*: improve mail template and portal view

_*= account, hr_timesheet
This commit improves the Portal functionality and add email template:

- improve email template for invoice includes timesheet (add link in mail which
  redirect to timesheets linked with invoice).
- improve timesheets template by renaming labels and internal header/footer.

task-3251680

closes odoo/odoo#127224

Related: odoo/enterprise#43593
Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>

addons/account/data/mail_template_data.xml

@@ -45,6 +45,10 @@
                 Please use the following communication for your payment: <span style="font-weight:bold;" t-out="object.payment_reference or ''">INV/2021/05/0005</span>.
             </t>
         </t>
+        <t t-if="hasattr(object, 'timesheet_count') and object.timesheet_count">
+            <br /><br />
+            PS: you can review your timesheets <a t-att-href="'my/timesheets?search_in=invoice&amp;search=%s' % object.name">from the portal.</a>
+        </t>
         <br /><br />
         Do not hesitate to contact us if you have any questions.
         <t t-if="not is_html_empty(object.invoice_user_id.signature)">