Participants: fix table layout for read-only users.

The content-cell condition did not match the heading-cell condition.
The result was that the "invited by" column was missing and in invalid markup:
![bildschirmfoto 2014-08-07 um 13 41 53](https://cloud.githubusercontent.com/assets/7469/3841148/dc7053a2-1e27-11e4-8cf4-21eaf3245281.png)

@maethu