Upload crashes to collab directly (#8649)

This lets us run rustc_demangle on the backtrace, which helps the Slack
view significantly.

We're also now uploading files to digital ocean's S3 equivalent (with a
1 month expiry) instead of to Slack.

This PR paves the way for (but does not yet implement) sending this data
to clickhouse too.

Release Notes:

- N/A

crates/collab/k8s/collab.template.yml

@@ -156,6 +156,11 @@ spec:
                 secretKeyRef:
                   name: clickhouse
                   key: database
+            - name: SLACK_PANICS_WEBHOOK
+              valueFrom:
+                secretKeyRef:
+                  name: slack
+                  key: panics_webhook
             - name: INVITE_LINK_PREFIX
               value: ${INVITE_LINK_PREFIX}
             - name: RUST_BACKTRACE