ProjectTagsScreen: Fix title of revision field for new tag

The UI for tag creation was based on the existing UI for branch
creation, and the field is named "Initial Revision".

"Initial Revision" makes sense for branches, where the revision
of the branch may advance as changes are submitted.

However, it does not make sense for tags, whose revision will
not change.

Change the title to only "Revision".

Change-Id: I5c5ee711da7ef196f8a7398f5b479f8c0f31d302

gerrit-gwtui/src/main/java/com/google/gerrit/client/admin/AdminConstants.java

@@ -179,6 +179,8 @@ public interface AdminConstants extends Constants {
 
   String initialRevision();
 
+  String revision();
+
   String buttonAddBranch();
 
   String buttonDeleteBranch();