fix: Fix button alignment for email templates in Outlook

The button group generated in email templates is expected to show the
two buttons side by side in a single row, but in Outlook both buttons
took the full width of the wrapper row and each button was shown in
its own row. To solve that the buttons are wrapped in an additional
table that shows each button in its own cell, limiting their width and
showing them in a single row; this is done conditionally and only
applied in Outlook, so it should not affect other clients.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

Author: danxuliu