Add check for changed password

These tests are incomplete, but I have no idea how to write the other
half.

src/documents/checks.py

@@ -0,0 +1,26 @@
+from django.core.checks import Warning, register
+
+
+@register()
+def changed_password_check(app_configs, **kwargs):
+
+    from documents.models import Document
+    from paperless.db import GnuPG
+
+    warning = (
+        "At least one document:\n\n  {}\n\nin your data store was encrypted "
+        "with a password other than the one currently\nin use.  This means "
+        "that this file, and others encrypted with the other\npassword are no "
+        "longer acessible, which is probably not what you want.  If\nyou "
+        "intend to change your Paperless password, you must first export all "
+        "of\nthe old documents, start fresh with the new password and then "
+        "re-import them."
+    )
+
+    document = Document.objects.order_by("-pk").filter(
+        storage_type=Document.STORAGE_TYPE_GPG
+    ).first()
+
+    if document and not GnuPG.decrypted(document.source_file):
+        return [Warning(warning.format(document))]
+    return []