Tweaks to captcha configuration

[dread]

Awesome to have the captcha implementation and running on thedatahub.org! Here are some further suggestions concerning the config options, before this goes into a release:

• Document "ckan.recaptcha.publickey" and "ckan.recaptcha.privatekey" in doc/configuration.rst
• Which one config option defines whether recaptcha is enabled? There is disagreement between the template and the lib. Maybe better to have a third option for this. When setting the syntax for this, I can see us wanting to use captcha for other operations, like package edit, package create, group create etc. so perhaps we'd want to be able to turn each one on/off separately, using the name of the logic action. e.g. ckan.recaptcha.enable = user_create, group_create
• We got this exception last night from the template:

  Module ?:37 in <Expression u'g.recaptcha_publickey'>
  <<          <dd><input type="password" name="password2" value="" /></dd>
         
                 <dd py:if="g.recaptcha_publickey">
                   <script type="text/javascript"
                     src="http://www.google.com/recaptcha/api/challenge?k=${g.recaptcha_publickey}">
  >>  <dd py:if="g.recaptcha_publickey">
  Module genshi.template.eval:324 in lookup_attr
  <<                      val = obj[key]
                         except (KeyError, TypeError):
                             val = cls.undefined(key, owner=obj)
                 return val
  >>  val = cls.undefined(key, owner=obj)
  Module genshi.template.eval:410 in undefined
  <<          """Raise an ``UndefinedError`` immediately."""
                 __traceback_hide__ = True
                 raise UndefinedError(key, owner=owner)
  >>  raise UndefinedError(key, owner=owner)
  UndefinedError: <ckan.lib.app_globals.Globals object at 0xb8f1182c> has no member named "recaptcha_publickey"
  

I'm not sure why - might be something to do with the maintenance on the server, but worth double checking, since we don't want exceptions, say if one or other config variable is not defined.

[rgrp]

Error was due to maintenance. Need both config options for recaptcha to work so either one can be used to check config. Re docs see #1358 (auto generation of docs on config from the config source).

Given that only docs remain and that is part of #1358 am closing as duplicate.