Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

app/controllers/api/v1/accounts/integrations/slack_controller.rb

@@ -0,0 +1,36 @@
+class Api::V1::Accounts::Integrations::SlackController < Api::V1::Accounts::BaseController
+  before_action :fetch_hook, only: [:update, :destroy]
+
+  def create
+    builder = Integrations::Slack::HookBuilder.new(
+      account: current_account,
+      code: params[:code],
+      inbox_id: params[:inbox_id]
+    )
+
+    @hook = builder.perform
+
+    render json: @hook
+  end
+
+  def update
+    builder = Integrations::Slack::ChannelBuilder.new(
+      hook: @hook, channel: params[:channel]
+    )
+    builder.perform
+
+    render json: @hook
+  end
+
+  def destroy
+    @hook.destroy
+
+    head :ok
+  end
+
+  private
+
+  def fetch_hook
+    @hook = Integrations::Hook.find(params[:id])
+  end
+end