[FIX] various: avoid scroll for button(s) within mail templates

Right now, for notifications handled by odoo (which are being displayed on
discuss app / object chatter), few of the buttons have unexpected scroll
(mostly the CTAs that redirect to record / portal views) and ruins the UI.

This commit fixes the issue by improving such templates by using proper
paddings on the button.

TaskId:2518863

closes odoo/odoo#70083

Related: odoo/enterprise#18225
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mail/data/mail_templates.xml

@@ -270,7 +270,7 @@
     <span>Dear <t t-esc="object.user_id.sudo().name"/>,</span><br />
     <span style="margin-top: 8px;">You have been assigned to the <t t-esc="model_description or 'document'"/> <t t-esc="object.display_name"/>.</span>
 </p>
-<p style="margin-top: 24px; margin-bottom: 16px;">
+<p style="padding-top: 24px; padding-bottom: 16px;">
     <a t-att-href="access_link" t-att-data-oe-model="object._name" t-att-data-oe-id="object.id" style="background-color:#875A7B; padding: 10px; text-decoration: none; color: #fff; border-radius: 5px;">
             View <t t-esc="model_description or 'document'"/>
     </a>
@@ -303,7 +303,7 @@
     <t t-if="activity.summary">(<span t-field="activity.summary"/>)</t>
     on <span t-field="activity.res_name"/>
     to close for <span t-field="activity.date_deadline"/>.<br />
-    <p style="margin: 16px 0px 16px 0px;">
+    <p style="padding: 16px 0px 16px 0px;">
         <a t-att-href="access_link" t-att-data-oe-model="activity.res_model" t-att-data-oe-id="activity.res_id"
             style="background-color:#875A7B; padding: 8px 16px 8px 16px; text-decoration: none; color: #fff; border-radius: 5px;">
             View <t t-esc="model_description or 'document'"/>