Add option to API to update PullRequest base branch (#11666) (#11796)

* EditPull: add option to change base

Close #11552

Co-authored-by: Lauris BH <lauris@nix.lv>

templates/swagger/v1_json.tmpl

@@ -6593,6 +6593,9 @@
           "403": {
             "$ref": "#/responses/forbidden"
           },
+          "409": {
+            "$ref": "#/responses/error"
+          },
           "412": {
             "$ref": "#/responses/error"
           },
@@ -11977,6 +11980,10 @@
           },
           "x-go-name": "Assignees"
         },
+        "base": {
+          "type": "string",
+          "x-go-name": "Base"
+        },
         "body": {
           "type": "string",
           "x-go-name": "Body"