feat(import): disable back button during loading and fix spacing in upload error message

Signed-off-by: Manuel Abascal <mjabascal10@gmail.com>

Author: mjabascal10

frontend/src/app/rule-management/app-rule/components/import-rules/import-rule.component.html

@@ -210,6 +210,7 @@
 
     <!-- BACK -->
     <button *ngIf="currentStep !== RULE_FORM.STEP0 && !isImportFinished"
+            [disabled]="loading"
             (click)="back()"
             class="btn utm-button utm-button-primary mr-2">
       <i class="icon-arrow-left32"></i> Back

frontend/src/app/shared/components/utm/util/utm-file-upload/utm-file-upload.component.html

@@ -6,7 +6,7 @@ <h6 class="font-weight-light text-blue-800">Drag file or click here to select {{
          hidden
          type="file">
   <small *ngIf="maxFilesError" class="form-text text-danger">
-    {{ 'You can upload up to ' + maxFiles + 'files only.'}}
+    {{ 'You can upload up to ' + maxFiles + ' files only.'}}
   </small>
 </div>