chore: Introduce enterprise edition license (#3209)

- Initialize an "enterprise" folder that is copyrighted.
- You can remove this folder and the system will continue functioning normally, in case you want a purely MIT licensed product.
- Enable limit on the number of user accounts in enterprise code.
- Use enterprise edition injector methods (inspired from Gitlab).
- SaaS software would run enterprise edition software always.

Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

db/migrate/20211012135050_add_limits_to_accounts.rb

@@ -0,0 +1,5 @@
+class AddLimitsToAccounts < ActiveRecord::Migration[6.1]
+  def change
+    add_column :accounts, :limits, :jsonb, default: {}
+  end
+end