Migrate Discord and Slack Webhooks to ManageWiki (#3689)

• Migrate Discord and Slack Webhooks to ManageWiki

• Rename variable to wmg

• Also add original variable

• New visibility key format

• New visibility key format

• Add wmgDiscordAdditionalIncomingWebhookUrl

• Rename variable

• Rename variable

• Unrestrict SlackNotifications

• Changs defaults

• Add wgSlackIncomingWebhookUrl

• Add Discord webhook url value; rename config

• fix spacing

• fix wgDiscordAdditionalIncomingWebhookUrls

• use original variable

• use original variable

• fix config name