models: Enforce stricter requirements on the full_name field.

This changes the requirements for UserProfile to disallow some
additional characters, with the overall goal of being able to use
formataddr in send_mail.py.

We don't need to be particularly careful in the database migration,
because user full_names are not required to be unique.

zerver/migrations/0236_remove_illegal_characters_email_full.py

@@ -0,0 +1,31 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.20 on 2019-06-28 21:45
+from __future__ import unicode_literals
+
+from django.db import migrations
+from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+from unicodedata import category
+
+NAME_INVALID_CHARS = ['*', '`', "\\", '>', '"', '@']
+
+
+def remove_name_illegal_chars(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    UserProfile = apps.get_model("zerver", "UserProfile")
+    for user in UserProfile.objects.all():
+        stripped = []
+        for char in user.full_name:
+            if (char not in NAME_INVALID_CHARS) and (category(char)[0] != "C"):
+                stripped.append(char)
+        user.full_name = "".join(stripped)
+        user.save(update_fields=["full_name"])
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0235_userprofile_desktop_icon_count_display'),
+    ]
+
+    operations = [
+        migrations.RunPython(remove_name_illegal_chars)
+    ]