Added option to disable migrations

This patch introduces DISABLE_MIGRATIONS parameter in [repository]
section of app.ini (by default set to false). If set to true
it blocks access to repository migration feature.

This mod hides also local repo import option in user editor if
local repo importing or migrations is disabled.

Author-Change-Id: IB#1105130

modules/cron/tasks_basic.go

@@ -115,5 +115,7 @@ func initBasicTasks() {
 	registerArchiveCleanup()
 	registerSyncExternalUsers()
 	registerDeletedBranchesCleanup()
-	registerUpdateMigrationPosterID()
+	if !setting.Repository.DisableMigrations {
+		registerUpdateMigrationPosterID()
+	}
 }