enable "soft limits" for mod_choice

The Choice activity allows the teacher to set a "limit" to the number of answers to each option.

This proposes to implement a "soft limit"

 - that is disabled (by default) at the site level

 - that (like "limit") can be activated in each activity instance (disabled by default)

 - that (like "limit") can be set independently for each choice option

As such, a soft limit is not enforced, but answers made after the soft limit for an option has been reached are displayed in a different color.