Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

app/views/api/v1/accounts/integrations/apps/index.json.jbuilder

@@ -0,0 +1,7 @@
+json.array! @apps do |app|
+  json.id app.id
+  json.name app.name
+  json.logo app.logo
+  json.enabled app.enabled?(@current_account)
+  json.button app.button
+end

app/views/api/v1/accounts/integrations/apps/show.json.jbuilder

@@ -0,0 +1,7 @@
+json.id @app.id
+json.name @app.name
+json.logo @app.logo
+json.description @app.description
+json.fields @app.fields
+json.enabled @app.enabled?(@current_account)
+json.button @app.button