UI improvements 2

• on page /admin/professor/{username} make ui improvements
• on page /admin/professor/{username}/edit there is no button for Back
• make sure that the dropdown enum values are normalized everywhere in the project (for example: Teaching assistant instead of TEACHING_ASSISTANT)
• on page /admin/subject/statistics normalize the pagination and results per page
• on page /admin/study-programs, the default accreditation filter is 2018, but it should be 2023 (or the newest accreditation year), also sort the years in descending order (this is on page /admin/accreditations)
• on that same page normalize the buttons padding/margins