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: Remote Roster Management
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Introduction
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This module adds support for [XEP-0321: Remote Roster Management] which
is commonly used to allow components such as transports to modify the
rosters of local users.