Deprecate Confluence-side plugin in favor of webhooks

[mickmister]

Currently, a sysadmin must install both a plugin on Mattermost and Confluence in order to integrate the two systems together. Confluence recently introduced webhooks for their server platform. We can use these webhooks instead of requiring a plugin to be installed on Confluence.

https://confluence.atlassian.com/doc/managing-webhooks-1021225606.html

Issue created from a Mattermost message by @levb.