security-checker gadget

add sensiolabs security checker.

https://security.sensiolabs.org/

i added a new database column `description` in the issue table. 
i think it is a good idea to allow markdown, isn't it?

waiting for #65 because of migrations.
Should we make this configurable? Of course, failed security checks are a bad thing, but they do not even really correspond to the commit, that triggers the issue. And of course, e.g. internal projects might not consider this critical errors.