luci-base: fix cbi dropdown quirks with MS Edge

On MS Edge, the behaviour of "value" attributes on "li" elements is
unreliable, so use the "data-" prefix to circumvent the problem.

Ref: https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/19320991/
Fixes: #2224
Signed-off-by: Jo-Philipp Wich <jo@mein.io>