Finished simple Spring Security implementation

- Added a SecurityFilterChain in the GotItInitializer to properly handle
  requests for the Spring Security system
- Modified database to add "authorities" table for users roles purpose
- Refactoring of all database tables and entities
  -> Changed naming convention for PK and composite keys from
     "id_<entity>" to "pk_<entity>"
  -> Changed naming convention for FK from "id_<entity>" to "fk_<entity>"
  -> New convention: all PK, composite keys and FK are integers (no more
     VARCHAR(x) as PK)
  -> In entities: changed naming convention for PK from "id" to
     "<entity>Pk"
  -> In entities: embeddable keys now correctly follow the UpperCamelCase
     convention (e.g. from "AssessmentStudentPK" to "AssessmentStudentPk"
- Created an ErrorController for errors pages handling purpose, such as
  403 or 404
- Created an AdminController for future administration panel
- Renamed some entities to better fit their roles (and their related PK):
  -> From "EmployeeSchoolEntity" to "EmployeeSchoolTypeEntity"
  -> From "AssessmentResultEntity" to "AssessmentStudentEntity"
- Overrided "create" service for UserService (activation + role auto.)
- Added new line in some files where it was missing
- Added simple views for errors 403 and 404
- Added named query to find authority by role