Initial entry for Slack Enterprise (#804)

Omitted 'Grid' from the name since it will probably get cut-off and look like "Slack Enterprise Gr...", also it's not super relevant for end users.

apps.yml

@@ -4264,3 +4264,15 @@ apps:
     name: Nemo Grafana
     op: auth0
     url: https://nemo.mozilla.net/grafana/
+- application:
+    authorized_groups: []
+    authorized_users:
+    - [email protected]
+    - [email protected]
+    client_id: htnpYVQ6jsBnxstKeETNerYgVUjvE7SB
+    display: false
+    logo: slack.png
+    name: Slack Enterprise
+    op: auth0
+    url: https://auth.mozilla.auth0.com/samlp/htnpYVQ6jsBnxstKeETNerYgVUjvE7SB
+    vanity_url: