Merge branch 'M2-exercise' into M2

# Conflicts:
#	application/model/src/main/java/org/fuseri/model/dto/course/CourseDto.java
#	application/model/src/main/java/org/fuseri/model/dto/lecture/LectureDto.java
#	application/model/src/main/java/org/fuseri/model/dto/user/UserDto.java
#	application/module-certificate/src/test/java/org/fuseri/modulecertificate/CertificateControllerTests.java
#	application/module-language-school/src/test/java/org/fuseri/modulelanguageschool/course/CourseControllerTest.java
#	application/module-language-school/src/test/java/org/fuseri/modulelanguageschool/user/UserControllerTest.java