Update Thymeleaf to 3.0.15

[php-coder]

• 3.0.12
• 3.0.13
• 3.0.14
• 3.0.15

See https://www.thymeleaf.org/releasenotes.html

[php-coder]

Series.Add-Comment.Logic.Add a comment test fails with exception:

ParseException: Instantiation of new objects and access to static classes is forbidden in this context (template: "series/info" - line 393, col 29)

Here is the related issue: thymeleaf/thymeleaf#816