[TOOLING] Add Automation for When a PR or Issue is Assigned (#10191)

Adding automation for team triage meetings for when PRs or Issues are
assigned. You can see an example in the "Test" channel.

---------

Co-authored-by: ohemorange <erica@eff.org>

.github/workflows/assigned.yaml

@@ -0,0 +1,17 @@
+name: Issue Assigned
+
+on:
+  issues:
+    types: [assigned]
+
+  pull_request:
+     types: [assigned]
+jobs:
+  send-mattermost-message:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: mattermost/action-mattermost-notify@master
+      with:
+        MATTERMOST_WEBHOOK_URL: ${{ secrets.MATTERMOST_ASSIGN_WEBHOOK }}
+        TEXT: >
+            ${{ github.event.issue.assignee.login || github.event.pull_request.assignee.login }} assigned to "${{ github.event.issue.title || github.event.pull_request.title }}": ${{ github.event.issue.html_url || github.event.pull_request.html_url }}