Store initial value for boolean selects in data-initial-value too

This was done in 02ceddb for other selects, but boolean ones were
forgotten.

Note that we need a handlebars helper to properly store "false". See
http://stackoverflow.com/questions/17664323/handlebars-doesnt-render-boolean-variables-when-false
Merge pull request #1107 from vuntz/initial-value-boolean

Store initial value for boolean selects in data-initial-value too