#560 - Fix server config validation issues.

Fixes:

1. Test Email functionality in server configuration page.
2. Validate LDAP functionality in server configuration page.
3. Reload Command Repo Cache functionality in server configuration page.

All were broken because CSRF token was not being sent, and in Rails 4, even
AJAX requests are checked for CSRF. Wasn't the case with Rails 2.
Two things about it:

1. session is a hash. I think it should be fine to read twice from it.

2. Putting a form_tag around the button will put the form_authenticity_token, like you said and form.serialize() will send it along as well. However, the button data is not the only thing that needs to get sent. Parts of the bigger (outside) form needs to get sent as well. For "Test Email" button, it is all of the email related [input element information](https://github.com/gocd/gocd/blob/master/server/webapp/WEB-INF/rails.new/app/assets/javascripts/server_configuration.js#L71). For "Validate LDAP", it is information from the part of the bigger form [related to LDAP](https://github.com/gocd/gocd/blob/master/server/webapp/WEB-INF/rails.new/app/assets/javascripts/server_configuration.js#L95). For the reload command repo cache, you'd be right, since there's no other form information that is being sent. But then, in the same page, there would be inconsistent ways of sending information to the server. I'd rather just keep it the same.

Right?