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

conf/provisioning/alerting/sample.yaml

@@ -23,7 +23,7 @@ apiVersion: 1
 #       #                  evaluation - should be obtained via the API
 #       data:
 #       - refId: A
-#         datasourceUid: "-100"
+#         datasourceUid: "__expr__"
 #         model:
 #           conditions:
 #           - evaluator:
@@ -40,7 +40,7 @@ apiVersion: 1
 #             type: query
 #           datasource:
 #             type: __expr__
-#             uid: "-100"
+#             uid: "__expr__"
 #           expression: 1==0
 #           intervalMs: 1000
 #           maxDataPoints: 43200