fix some problems

[newnewcoder]

1. flywaydb downgrade to support mysql5.7
2. remove ThymeleafConfig according to https://ultraq.github.io/thymeleaf-layout-dialect/migrating-to-3.0/#automatic-module-name-and-package-restructure
3. remove junit4 annotations
4. add th:remove="tag" to prevent header going off