Convert files to typescript

The change converts the following files to typescript:

* elements/settings/gr-settings-view/gr-settings-view.ts

Change-Id: I758f919418b98d077efeb2ea42d23908e064e216
diff --git a/polygerrit-ui/app/constants/constants.ts b/polygerrit-ui/app/constants/constants.ts
index 2db076f..b326001 100644
--- a/polygerrit-ui/app/constants/constants.ts
+++ b/polygerrit-ui/app/constants/constants.ts
@@ -296,6 +296,16 @@
 }
 
 /**
+ * The type of email format to use.
+ * Doesn't mentioned in doc, but exists in Java class GeneralPreferencesInfo.
+ */
+
+export enum EmailFormat {
+  PLAINTEXT = 'PLAINTEXT',
+  HTML_PLAINTEXT = 'HTML_PLAINTEXT',
+}
+
+/**
  * The base which should be pre-selected in the 'Diff Against' drop-down list when the change screen is opened for a merge commit
  * https://gerrit-review.googlesource.com/Documentation/rest-api-accounts.html#preferences-input
  */