Fixed #1326: Rearranged assignment pdf table cells to prevent a LayoutError

This is a 2nd fix in this issue. Initally it looks like the error was caused by >20 posts only. But same LayoutError also affected if you added more than 20 candiadates to an assignment.
Now it is fixed by rearranged table (every candidate in one cell/row) to prevent that one cell is not higher than one page.

Note: The main diff of this pull request is caused by new indentation (``if posts:``) and rename cell name. 
Please review and merge.