Prevents the silencing of notifications that are actively deployed. (#454)

* Renaming 'active' to 'notify' as this is clearer and more aligned to what this value is actually controlling. 'active' is now a property that depends on whether any endpoints were found to be using the certificate. Also added logic for issue #405 disallowing for a certificates' notifications to be silenced when it is actively deployed on an endpoint.

* Adding migration script to alter 'active' column.

lemur/migrations/versions/932525b82f1a_.py

@@ -0,0 +1,21 @@
+"""empty message
+
+Revision ID: 932525b82f1a
+Revises: 7f71c0cea31a
+Create Date: 2016-10-13 20:14:33.928029
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '932525b82f1a'
+down_revision = '7f71c0cea31a'
+
+from alembic import op
+
+
+def upgrade():
+    op.alter_column('certificates', 'active', new_column_name='notify')
+
+
+def downgrade():
+    op.alter_column('certificates', 'notify', new_column_name='active')