optimize and migrate Disease data

disease rows are now unique and managed with a manytomany field
fixes #19

ippisite/ippidb/migrations/0013_auto_20170518_1943.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.1 on 2017-05-18 19:43
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('ippidb', '0012_auto_20170518_1913'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='disease',
+            name='disease_name',
+        ),
+        migrations.RemoveField(
+            model_name='disease',
+            name='ppi',
+        ),
+        migrations.AddField(
+            model_name='disease',
+            name='name',
+            field=models.CharField(default=None, max_length=30, unique=True, verbose_name='Disease'),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='ppi',
+            name='diseases',
+            field=models.ManyToManyField(to='ippidb.Disease'),
+        ),
+    ]