Details
Description
Add https://github.com/Gregwar/Captcha to the composer file (and thus vendor code).
Add requirement for the GD library to Blesta's recommended installation requirements.
Add a new extension of Blesta/Core/Util/Captcha/Common/AbstractCaptcha called InternalCaptcha. See Blesta/Core/Util/Captcha/Captchas/ReCaptcha for an example.
We already recommend mbstring in Blesta, though likely we should update the description to mention captcha support.