[ENH] Newsletter - Subscribe plugin: added an option to disable the captcha...

[ENH] Newsletter - Subscribe plugin: added an option to disable the captcha feature for anonymous, default and previous behaviour unchanged.

When captcha is enable for anonymous the visitor has to fill a captcha to subscribe to a newsletter. It is not super user friendly and there are case admin may need to allow visitors to subscribe without filling a captcha. This commit add a parameter to use the subscribe newsletter wikiplugin without captcha.