Resolve "Previews for importing"

• Added previews for improvement for importing grades and importing people.
• Shows a max of 5 rows as preview.
• Makes top row blue if has headers is checked else it is normal (made default as has headers)
• User is informed of grading and staff/students importing errors.

Closes #37 (closed)

src/main/java/nl/tudelft/submit/controller/AssignmentController.java

@@ -44,6 +44,7 @@ import org.springframework.validation.annotation.Validated;
 import org.springframework.web.bind.annotation.*;
 import org.springframework.web.multipart.MultipartFile;
 import org.springframework.web.multipart.MultipartHttpServletRequest;
+import org.springframework.web.servlet.mvc.support.RedirectAttributes;
 
 import com.opencsv.exceptions.CsvValidationException;
 
@@ -72,6 +73,7 @@ import nl.tudelft.submit.dto.view.VersionViewDTO;
 import nl.tudelft.submit.dto.view.labracore.SubmitAssignmentDetailsDTO;
 import nl.tudelft.submit.dto.view.labracore.SubmitSubmissionViewDTO;
 import nl.tudelft.submit.dto.view.script.ScriptTrainResultViewDTO;
+import nl.tudelft.submit.io.ImportHandler;
 import nl.tudelft.submit.model.Signature;
 import nl.tudelft.submit.model.TestSubmission;
 import nl.tudelft.submit.security.AuthorizationService;
@@ -365,10 +367,13 @@ public class AssignmentController {
 	@PostMapping("{id}/import/grades")
 	@PreAuthorize("@authorizationService.canImportAssignmentGrades(#id)")
 	public String importGrades(@PathVariable Long id, @RequestParam("file") MultipartFile file,
-			@ModelAttribute("gradeImport") AssignmentGradeImportDTO gradeImport)
-			throws IOException, CsvValidationException {
-		assignmentService.importGrades(id,
+			@ModelAttribute("gradeImport") AssignmentGradeImportDTO gradeImport,
+			RedirectAttributes redirectAttributes) throws IOException, CsvValidationException {
+		ImportHandler.Result<GradeImportDTO> result = assignmentService.importGrades(id,
 				new InputStreamReader(file.getInputStream(), StandardCharsets.UTF_8), gradeImport);
+		redirectAttributes.addFlashAttribute("successfulImportSize", result.result().size());
+		redirectAttributes.addFlashAttribute("importWarnings", result.warnings());
+		redirectAttributes.addFlashAttribute("importErrors", result.errors());
 		return "redirect:/assignment/{id}";
 	}