Allow notification of two reviewers being assigned to a PR and two issue assignees (#10345)

Fixes https://github.com/certbot/certbot/issues/10344

You can see this working in the mattermost "Test" channel, where I ran
this code from my test repo.

The documentation for the PR reviewer syntax is here:
https://docs.github.com/en/webhooks/webhook-events-and-payloads?actionType=review_requested#pull_request

We now no longer notify on PR assignees. But I think that is the correct
behavior.

This PR also fixes a bug in the issue assigned notification code, and
now lets you see when two different people were assigned. That
documentation is here:
https://docs.github.com/en/webhooks/webhook-events-and-payloads#

After this is in, I'll make the same changes to the josepy repo.

You can see the `if` syntax here:
https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows

```
on:
  pull_request:
    types: [review_requested]
jobs:
  specific_review_requested:
    runs-on: ubuntu-latest
    if: ${{ github.event.requested_team.name == 'octo-team'}}
    steps:
      - run: echo 'A review from octo-team was requested'
```

---------

Co-authored-by: Brad Warren <bmw@users.noreply.github.com>

.github/workflows/assigned.yaml

@@ -3,9 +3,6 @@ name: Issue Assigned
 on:
   issues:
     types: [assigned]
-
-  pull_request_target:
-     types: [assigned]
 jobs:
   send-mattermost-message:
     runs-on: ubuntu-latest
@@ -14,4 +11,4 @@ jobs:
       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 }}
+            ${{ github.event.assignee.login }} assigned to "${{ github.event.issue.title }}": ${{ github.event.issue.html_url }}

.github/workflows/review_requested.yaml

@@ -0,0 +1,16 @@
+name: Review Requested
+
+on:
+  pull_request_target:
+    types: [review_requested]
+jobs:
+  send-mattermost-message:
+    # Don't notify for the interim step of certbot/eff-devs being assigned
+    if: ${{ github.event.requested_reviewer.login != ''}}
+    runs-on: ubuntu-latest
+    steps:
+    - uses: mattermost/action-mattermost-notify@master
+      with:
+        MATTERMOST_WEBHOOK_URL: ${{ secrets.MATTERMOST_ASSIGN_WEBHOOK }}
+        TEXT: >
+            Review requested from ${{ github.event.requested_reviewer.login }} for "${{ github.event.pull_request.title }}": ${{ github.event.pull_request.html_url }}