Lock the repository to block deploying. (#84)

* Add `locked` field to Repo

* Add validate locked in deployments API

* Add lock & unlock button in settings

Author: Noah Hanjun Lee

docs/concepts/deployment.md

@@ -10,6 +10,8 @@ With approval, Gitploy waits until it matches the required approving approvals.
 
 ## Rollback
 
-Rollback is the best way to recover while you fix the problems. Gitploy supports the rollback. You can choose one of the successful deployments to rollback. 
+Rollback is the best way to recover while you fix the problems. Gitploy supports the rollback, and you can choose one of the successful deployments to rollback. 
+
+For best practice, you should lock the repository to block deploying by others. And the admin user has to take care of the repository until finishing to fix the problems.
 
 *Note that if the ref of the selected deployment is a branch, Gitploy automatically references the commit SHA to prevent deploying the head of the branch.*

docs/concepts/permission.md

@@ -13,6 +13,6 @@ Here are capabilities for each permission:
 
 * **Write** - Users can deploy and rollback for the `REF`. 
 
-* **Admin** - Users can modify repository settings, including activate and deactivate. 
+* **Admin** - Users can configures the repository, such as activating and locking.
 
 Of course, write and admin permission cover the ability of read permission.