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-httpd/src/main/java/com/google/gerrit/httpd/GerritConfigProvider.java

@@ -106,6 +106,7 @@ class GerritConfigProvider implements Provider<GerritConfig> {
       case OPENID_SSO:
         break;
     }
+    config.setSwitchAccountUrl(cfg.getString("auth", null, "switchAccountUrl"));
     config.setUseContributorAgreements(cfg.getBoolean("auth",
         "contributoragreements", false));
     config.setGitDaemonUrl(cfg.getString("gerrit", null, "canonicalgiturl"));