Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

lib/integrations/slack/hook_builder.rb

@@ -0,0 +1,42 @@
+class Integrations::Slack::HookBuilder
+  attr_reader :params
+
+  def initialize(params)
+    @params = params
+  end
+
+  def perform
+    token = fetch_access_token
+
+    hook = account.hooks.new(
+      access_token: token,
+      status: 'enabled',
+      inbox_id: params[:inbox_id],
+      hook_type: hook_type,
+      app_id: 'cw_slack'
+    )
+
+    hook.save!
+    hook
+  end
+
+  private
+
+  def account
+    params[:account]
+  end
+
+  def hook_type
+    params[:inbox_id] ? 'inbox' : 'account'
+  end
+
+  def fetch_access_token
+    client = Slack::Web::Client.new
+
+    client.oauth_access(
+      client_id: ENV.fetch('SLACK_CLIENT_ID', 'TEST_CLIENT_ID'),
+      client_secret: ENV.fetch('SLACK_CLIENT_SECRET', 'TEST_CLIENT_SECRET'),
+      code: params[:code]
+    )['bot']['bot_access_token']
+  end
+end