Add REST endpoint to get info about server configuration

Some of the Gerrit configuration parameters can now be accessed by

  GET /config/server/info

This REST endpoint can also be used anonymously.

For now the REST endpoint returns only a very limited set of
configuration parameters, which are needed by the Gerrit Mylyn
Connector. The result contains information about:

- auth type, editable account fields, if contributor agreements are
  used
- contact store
- download schemes, commands and archive formats
- All-Projects and All-Users project names

At the moment the Mylyn Gerrit Connector retrieves this information by
parsing the config from the HostPageData which is sent to the client.
This is an internal data structure which is used to exchange
information between Gerrit server and Gerrit WebUI. It's not an API
and third-party tools should not rely on it. As a result the Mylyn
Gerrit Connector is currently broken for Gerrit 2.11 and newer.

Provide the Mylyn Gerrit Connector team a stable API that provides
them the information they need so that future breakages can be
avoided. See Eclipse Bugzilla issue 465132 [1] for further details.

The structure of the returned JSON correlates to the structure in the
gerrit.config file.

[1] https://bugs.eclipse.org/bugs/show_bug.cgi?id=465132

Change-Id: Iac4be762bff971403438aa84923d9f0e11883366
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-server/src/main/java/com/google/gerrit/server/account/Realm.java

@@ -24,6 +24,9 @@ public interface Realm {
   /** Can the end-user modify this field of their own account? */
   public boolean allowsEdit(Account.FieldName field);
 
+  /** Returns the account fields that the end-user can modify. */
+  public Set<Account.FieldName> getEditableFields();
+
   public AuthRequest authenticate(AuthRequest who) throws AccountException;
 
   public AuthRequest link(ReviewDb db, Account.Id to, AuthRequest who)