Don't show ping URLs -- gmail converts them to clickable links, and people click on them.

Using new backend name which come with Django 1.9 django.db.backends.postgresql instead of django.db.backends.postgresql_psycopg2

Adding "healthcheck.io" as User-Agent to inform webhook handler server about request is specific from healthcheck.

Otherwise, if period=1m and grace=1m then we can end up generating huge useless tables.

Check.n_pings now stores the total number of pings the check has ever received. Running "prunepings" command doesn't affect this field. +a new "prunepingsslow" command which works in smaller chunks so is appropriate for initial pruning of a huge api_ping table.