[5.5] Default mail theme: Reset margins on th and td cells to fix Outlook propagation issue

[lupinitylabs]

Using the mail::table component and the default Mail theme, table rows rendered in Outlook appear with a 30px top and bottom margin around them, making them >60 pixel high each.

This is due to the margin: 30px auto declaration in the .table table selector being propagated to both .table th and .table td selectors.

This can be fixed by resetting margins in both .table thand .table td selectors without negative side-effects in other mail clients.

The issue and the fix were both tested and confirmed in Outlook 2016.

Before:

After applying the fix:

I might add that there are lots more issues with the default template in Outlook 2016, as some margin declarations are blatantly ignored and would have to be replaced by paddings instead to show any effect. But this should probably be up for discussion in a different pull request.