Add Switch Account link to the current user popdown

Permit site administrators to configure a URL for users to switch the
current user identity.  CacheBasedWebSession automatically invalidates
the old session (and the cookie and XSRF token) during the switch.

For DEVELOPMENT_BECOME_ANY_ACCOUNT this is a faster way to jump to
another test user as it avoids needing to logout first, saving one
click and a reload of the web UI.

For OPENID the user can select another OpenID user quickly using the
login dialog.  This can be useful if someone has two personas, such as
a personal identity used with open source work and another identity
used for day-job or administrative activities.

On gerrit-review the configuration setting will be set to facilitate
switching between a user's Google Accounts.

Change-Id: I8abd58c06e74f96fef5430324e1c24a787823d35

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

@@ -28,6 +28,7 @@ public class GerritConfig implements Cloneable {
   protected String registerText;
   protected String loginUrl;
   protected String loginText;
+  protected String switchAccountUrl;
   protected String httpPasswordUrl;
   protected String reportBugUrl;
   protected boolean gitBasicAuth;
@@ -75,6 +76,14 @@ public class GerritConfig implements Cloneable {
     registerUrl = u;
   }
 
+  public String getSwitchAccountUrl() {
+    return switchAccountUrl;
+  }
+
+  public void setSwitchAccountUrl(String u) {
+    switchAccountUrl = u;
+  }
+
   public String getRegisterText() {
     return registerText;
   }