[ticket/12155] Move existing button style to a more generic class.

PHPBB3-12155
[ticket/12155] Use the new .button class as the basis for the post buttons.

PHPBB3-12155
[ticket/12155] Remove obsolete images.

PHPBB3-12155
[ticket/12155] Remove unused font.

PHPBB3-12155
[ticket/12155] Clean up icons.

PHPBB3-12155