fix the ics invite

The invite wasn't working because the attendee line was
already quoted in the twig template, so it was double
quoted by the swiftmailer's quoted-printable option.

Before :
![before](https://f.cloud.github.com/assets/320372/463643/3cdb30ce-b586-11e2-86f1-81e4630e0366.png)

After :
![after](https://f.cloud.github.com/assets/320372/463644/41b37e76-b586-11e2-93ed-9cd837d982c4.png)