Mattermost Alert Backend

W.r.t. the user-contributed version, this:

* removes commented-out code
* removes channels (not supported at the UI level)
* removes all other things

Said differently: the mattermost Alert Service Backend is simply the Slack version
with edits to make it mattermost-specific (and nothing else).

(In a few places I've edited the slack backend to make comparing easier).

Fix #277
See #253

alerts/migrations/0004_alter_messagingserviceconfig_kind.py

@@ -0,0 +1,20 @@
+import alerts.models
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("alerts", "0003_messagingserviceconfig_last_failure_error_message_and_more"),
+    ]
+
+    # This is the "once and for all" migration since we depend on kinds_choices rather than a list now
+    operations = [
+        migrations.AlterField(
+            model_name="messagingserviceconfig",
+            name="kind",
+            field=models.CharField(
+                choices=alerts.models.kind_choices, default="slack", max_length=20
+            ),
+        ),
+    ]