Plugin architecture (#4861)

* wip

* Refactor, tasks, processors, routes loading

* Move Slack settings config to plugin

* Fix translations in plugins

* Move Slack auth to plugin

* test

* Move other slack-related files into plugin

* Forgot to save

* refactor

plugins/slack/plugin.json

@@ -0,0 +1,5 @@
+{
+  "name": "Slack",
+  "description": "Adds a Slack authentication provider, support for the /outline slash command, and link unfurling.",
+  "requiredEnvVars": ["SLACK_CLIENT_ID", "SLACK_CLIENT_SECRET"]
+}