Log Message:
Update contao35 to 3.5.20 which includes potential XSS security problem.

Version 3.5.20 (2016-12-19)
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### Fixed
Correctly show running repeated events in the event list (see #8588).

### Fixed
Improve the PHP 7.1 compatibility.

### Fixed
Keep the root nodes order in the page selector (see #8577).

### Fixed
Do not output invalid option values in widget error messages (see #8594).
Thanks to Pascal Gerundt for finding and reporting the issue.

### Fixed
Correctly parse english dates in MooTools (see #8573).