Log Message:
Update contao35 to 3.5.28.

Version 3.5.28 (2017-07-12)
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### Fixed
Prevent arbitrary PHP file inclusions in the back end (see CVE-2017-10993).

### Fixed
Improve the accessibility of the CAPTCHA widget (see #8709).

### Fixed
Fixed the iOS scrolling bug in the simple modal script (see #8708).

### Fixed
Correctly cache the unique keys in the SQL cache (see #8712).