UI and client support for hashtag manipulation

Add UI components to view/add/remove hashtags. The new
UI panel will be visible only if NoteDB is enabled.

Change-Id: Icb2c24bfc7f5d63d76f80ddcfb6b72fef02e370c

gerrit-common/src/main/java/com/google/gerrit/common/data/HostPageData.java

@@ -30,6 +30,7 @@ public class HostPageData {
   public Theme theme;
   public List<String> plugins;
   public List<Message> messages;
+  public boolean isNoteDbEnabled;
 
   public static class Theme {
     public String backgroundColor;