Update signature API to support uploads

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/document_signatures/models.py

@@ -43,7 +43,10 @@ class SignatureBaseModel(models.Model):
     date = models.DateField(
         blank=True, editable=False, null=True, verbose_name=_('Date signed')
     )
-    key_id = models.CharField(max_length=40, verbose_name=_('Key ID'))
+    key_id = models.CharField(
+        help_text=_('ID of the key that will be used to sign the document.'),
+        max_length=40, verbose_name=_('Key ID')
+    )
     # With proper key
     signature_id = models.CharField(
         blank=True, editable=False, null=True, max_length=64,
@@ -128,7 +131,9 @@ class EmbeddedSignature(SignatureBaseModel):
 @python_2_unicode_compatible
 class DetachedSignature(SignatureBaseModel):
     signature_file = models.FileField(
-        blank=True, null=True, storage=storage_detachedsignature,
+        blank=True, help_text=_(
+            'Signature file previously generated.'
+        ), null=True, storage=storage_detachedsignature,
         upload_to=upload_to, verbose_name=_('Signature file')
     )