Initial commit of mod_slack_webhooks. This provides an HTTP-based interface to and from Prosody-hosted MUCs equivalent to Slack's incoming and outgoing webhook interfaces, allowing a variety of Slack integrations to be used with a Prosody MUC.

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summary: Allow server administrators to probe any user
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This module lets server administrators send `<presence type="probe"/>`
to any local user and receive their presence in response, bypassing
roster checks.