Moved invite code to env var

* invite code is now set via env var `CONVOS_INVITE_CODE`.
* default invite code `convos` for docker container.
* no longer rendering the sign-up form (only to tell the submitter then, that he can't submit) if invite code is configured and provided one is wrong.

Oh and I also added an .editorconfig (http://editorconfig.org) file to not screw up your indentation. ;-)