Improve automatic emails with temporary urls

This MR targets the improvement for automatic emails concerning validation of new users by supervisors/re-validation of users.

Fixes #542 (closed)

Fixes #550 (closed)

beat/web/accounts/migrations/0013_supervisee_rejection_temporary_url.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.26 on 2020-05-13 18:48
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('accounts', '0012_accounts_temporary_url'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='temporaryurl',
+            name='status',
+            field=models.CharField(choices=[('V', 'Supervisor validates supervisee'), ('Y', 'Self yearly revalidation from supervisee'), ('R', 'Supervisor rejects supervisee revalidation')], default='V', max_length=1),
+        ),
+    ]