Allow shards to change the label of the "Account name" text field in the login dialog

[dgelessus]

Some shards expect the account name to be an e-mail address, whereas others expect an "account username" chosen by the user and won't recognize the underlying e-mail address (if any).

It would be nice if the "Account name" label in the login dialog could be changed to a more specific text for each shard. For example, for mystonline.com or Minkata it should say "Email address", whereas for Gehn or Destiny it should say "Forum username".

This could be implemented using a server.ini setting, for example Server.AccountNameLabel "Forum username". A free text string would cause localization issues in the future though, if we ever make the login dialog respect the user's chosen language. That could be solved either by only allowing fixed identifiers (accountName, emailAddress, forumUsername, ...) that get translated to a human-readable string by the client, or by adding a separate server.ini setting for each language (Server.AccountNameLabel.en "Forum username", Server.AccountNameLabel.de "Forum-Benutzername", ...).

Technically, shard operators can already do this by patching plClient.rc in the source code, but this is inconvenient and doesn't work for shards like Destiny that don't ship a customized client build.