Fixes: #348

Normally, when a webhook call fails (timeout, connection
error, non-2xx response), the HTTP request is retried up to two
times (so up to 3 times total). This is useful when sending
actual notifications, in case the webhook target has a temporary
glitch.

When interactively testing a webhook integration
("Send Test Notification" in the
"Integrations" page), we would prefer to see any errors ASAP
on the screen instead of retrying and so possibly swallowing them.

One specific use case is webhook targets that take long time to
generate a response. "Send Test Notification" is synchronous,
meaning that the user could be stuck for
5 x 3 = 15 seconds waiting for the  test HTTP request to time out
three times.

Fixes: #435

hc.lib.string.replace only replaces placeholders that appear in the original template. It ignores any placeholders that "emerge" while doing string substitutions. This is done mainly to avoid unexpected behavior when check names or tags contain dollar signs.

.Webhooks can have different request bodies and headers for "up" and "events".