[ticket/12533] Make <a> use all the container

Changed padding CSS to the notifications' `<li>` and it's
`<a>` (immediate child) so that the `<a>` block uses all
 it's container width and height.

To compensate that, I added the padding that was in the
`<li>` to the `<a>`

PHPBB3-12533
[ticket/12533] Make <a> use all the container

Changed the CSS so that the `<a>` of the notifications
list for the "see all" uses all the container.

PHPBB3-12533