cb_apply_filter should give more useful message on success

[ilevantis]

When a CBv1 cohort is used in cb_apply_filter() the function messages with the number of participants in the newly updated cohort. With a CBv2 cohort the number of participants is not given in a message.

The CBv2 API does not provide a response with the updated number of participants for PUT /cohort-browser/v2/cohort/{cohort_id}. Instead cb_participant_count() can be used to get this info.

Additionally, it would be useful to provide a web address for the cohort in the output of this (and other functions like cb_cohort_create() and cb_cohort_load()).