mail: keep mail backups separately from box backups

part of #717

src/backups.js

@@ -28,9 +28,11 @@ exports = module.exports = {
     testProviderConfig,
 
     BACKUP_IDENTIFIER_BOX: 'box',
+    BACKUP_IDENTIFIER_MAIL: 'mail',
 
     BACKUP_TYPE_APP: 'app',
     BACKUP_TYPE_BOX: 'box',
+    BACKUP_TYPE_MAIL: 'mail',
 
     BACKUP_STATE_NORMAL: 'normal', // should rename to created to avoid listing in UI?
     BACKUP_STATE_CREATING: 'creating',
@@ -238,6 +240,7 @@ function getSnapshotInfo(id) {
     return info[id] || { };
 }
 
+// keeps track of contents of the snapshot directory. this provides a way to clean up backups of uninstalled apps
 async function setSnapshotInfo(id, info) {
     assert.strictEqual(typeof id, 'string');
     assert.strictEqual(typeof info, 'object');