Add a global capability to allow generating HTTP passwords

The capability `generateHttpPassword` allows non-administrator
users to generate HTTP passwords for users other than self.

This capability would typically be assigned to a non-interactive
group to be able to register HTTP passwords for users from a
tool or web service that uses the Gerrit REST API.

Change-Id: I3d77cd406b0258d68cd0b8bb4dfbdcec97112450

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

@@ -58,6 +58,9 @@ public class GlobalCapability {
   /** Can flush any cache except the active web_sessions cache. */
   public static final String FLUSH_CACHES = "flushCaches";
 
+  /** Can generate HTTP passwords for user other than self. */
+  public static final String GENERATE_HTTP_PASSWORD = "generateHttpPassword";
+
   /** Can terminate any task using the kill command. */
   public static final String KILL_TASK = "killTask";