Flatten every form button, not only the submit input

[Fivell]

The defect

A custom button placed in an ActiveAdmin form keeps ActiveAdmin's stock
gradient, bevelled border and text shadow, sitting next to a flat themed
submit button. Reported in #25.

Cause

ActiveAdmin applies its dark-button mixin to three form elements at once:

form { input[type=submit], input[type=button], button { @include dark-button; } }

The theme's flat override named only form input[type=submit], plus the very
narrow fieldset.actions > ol > li.button_action > button. Anything else —
a plain <button>, an <input type="button"> — fell through to ActiveAdmin's
rule.

The change

One selector list, mirroring ActiveAdmin's own:

-form input[type=submit], a.button, a:link.button, a:visited.button, fieldset.actions > ol > li.button_action > button {
+form input[type=submit], form input[type=button], form button, a.button, a:link.button, a:visited.button {

The button_action selector is dropped because it is a strict subset of
form button, which now wins by source order.

Buttons outside a form are deliberately left alone: those are the dropdown
and table-tool controls the theme styles separately, not what the report is
about.

Before / after

A form whose fieldset.actions holds the normal submit next to a
<button type="button"> and an <input type="button">, neither carrying a
theme class.

Before:

After:

Screenshots are taken against a generated ActiveAdmin 3.5 admin. Of sixteen
scenarios shot across the two revisions, only the two form cases changed;
menu, title bar, index table, table tools, pagination, filter sidebar and
show page are byte-identical.

rake css passes: 8 override configurations compile clean, 6 bad ones
rejected.

Fixes #25