Expressions: Fixes the issue showing expressions editor (#62510)

* Use suggested value for uid

* update the snapshot

* use __expr__

* replace all -100 with __expr__

* update snapshot

* more changes

* revert redundant change

* Use expr.DatasourceUID where it's possible

* generate files

pkg/services/ngalert/api/test-data/test-rule.json

@@ -9,7 +9,7 @@
                     "from": 18000,
                     "to": 10800
                 },
-                "datasourceUid": "-100",
+                "datasourceUid": "__expr__",
                 "model": {
                     "type": "math",
                     "expression": "2 + 2 > 1"
@@ -18,4 +18,4 @@
         ],
         "now": "2021-03-11T14:50:42.218+02:00"
     }
-}
+}