start of external supplementary files

Preprints - Links to external supplementary files openlibhums#1590

src/repository/models.py

@@ -281,6 +281,12 @@ class Preprint(models.Model):
         null=True,
         on_delete=models.SET_NULL,
     )
+    submission_supplementary_files = models.ManyToManyField(
+        'PreprintSupplementaryFile',
+        related_name='submission_supplementary_files',
+        blank=True,
+        null=True,
+    )
     meta_image = models.ImageField(
         blank=True,
         null=True,
@@ -533,6 +539,11 @@ def download_url(self):
         )
 
 
+class PreprintSupplementaryFile(models.Model):
+    preprint = models.ForeignKey(Preprint)
+    url = models.URLField()
+
+
 class PreprintAccess(models.Model):
     preprint = models.ForeignKey(Preprint)
     file = models.ForeignKey(PreprintFile, blank=True, null=True)