[FIX] website_crm_partner_assign: fixing broken urls in mail template

Issue:
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In the lead forward mail template, there are some broken links, when we click on those links they are appended to the current url and redirect to a non-expected page.

Steps to reproduce:
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1. Go to CRM -> Create a new opportunity
2. Click on this new opportunity -> Assigned Partner
3. Select an assigned partner and click “Send Email”
4. Go to Settings -> Technical -> Emails
5. When clicking on “Partner Portal” it redirects to a non-existing page

Cause:
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The issue is happening because the urls are badly formatted and not using the correct tags

OPW-3074049

closes odoo/odoo#130300

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>