Move the document language choice list from the model to the form.

Pycoutry upgrades or user changes to the language choice list won't
trigger a migration anymore. Closes GitLab issue #328.

mayan/apps/documents/migrations/0037_auto_20161231_0617.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.4 on 2016-12-31 06:17
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('documents', '0036_auto_20161222_0534'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='document',
+            name='language',
+            field=models.CharField(blank=True, default='eng', max_length=8, verbose_name='Language'),
+        ),
+    ]