Allow optional local requests for outgoing webhooks

[Matvey-Kuk]

An OSS user reported in the community that he experienced problems with outgoing webhooks not being able to reach the desired host.

It happened that this security check didn't let it go: https://github.com/grafana/oncall/blob/dev/engine/apps/alerts/utils.py#L60

I propose to introduce env variable "ALLOW_DANGEROUS_WEBHOOKS" (false by default) for those who run OnCall in a trusted environment. This env variable will help to disable all webhook checks.

[Matvey-Kuk]

FYI: We're taking this to the main team's backlog.

[Matvey-Kuk]

Closing, since the feature is merged.