Restrict tag name characters

If a tag's name has some "uncommon" characters in it, like
parentheses (or probably others), it will cause errors for the
javascript which renders the event pages, since it tries to use
the tag name in the element id for the delete button.  We now require
that tags only use 0-9a-zA-Z_\- in their names. May have to go through
and delete some tags which use them.

gracedb/events/migrations/0022_restrict_tag_name_chars.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.5 on 2018-06-07 18:57
+from __future__ import unicode_literals
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0021_emobservation_N_not_editable'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='tag',
+            name='name',
+            field=models.CharField(max_length=100, unique=True, validators=[django.core.validators.RegexValidator(code=b'invalid_tag_name', message=b'Tag names can only include [0-9a-zA-z_-]', regex=b'^[0-9a-zA-Z_\\-]*$')]),
+        ),
+    ]

gracedb/events/models.py

@@ -808,7 +808,14 @@ class Tag(CleanSaveModel):
     track those things?  Doesn't seem like it.
     """
     name = models.CharField(max_length=100, null=False, blank=False,
-        unique=True)
+        unique=True,
+        validators=[
+            models.fields.validators.RegexValidator(
+                regex=r'^[0-9a-zA-Z_\-]*$',
+                message="Tag names can only include [0-9a-zA-z_-]",
+                code="invalid_tag_name",
+            )
+        ])
     displayName = models.CharField(max_length=200, null=True, blank=True)
 
     def __unicode__(self):