luci-theme-openwrt-2020: refining hide/show buttons

luci-theme-openwrt-2020: refining hide/show buttons

Create larger buttons for smaller
screens (same behaviour as
luci-theme-bootstrap).
Swap the "hide" and "show" button
styles to match all other luci-theme,
as "hide" is currently using primary
style and "show" is currently using
secondary style. Also, change mouse
icon from text cursor to pointer.
Slightly fade "show" button:
no variable for disabled opacity
like luci-theme-bootstrap,
assuming 0.7.
Remove fade when hovered.

Signed-off-by: Joshua Criss <redacted>