ci(GitHub-Actions): Grant minimum necessary scopes

Grant callable workflows the permissions they require. The Notify
Assignee and Notify Reviewers workflows require the contents:read scope.
actions/checkout uses the contents:read scope to check out the calling
repository. Granting this specific permission also has the effect of
reducing the scope of all unspecified permissions from read to none.
Since this repository is public, most of its data can be read without
additional permissions, but the contents:read scope is needed in the
callable workflows when the calling repository is private. Callable
workflows can only be called when granted the permissions they require.

.github/workflows/notify-assignee.yaml

@@ -7,6 +7,8 @@ jobs:
   notify-assignee:
     name: Notify Assignee
     uses: ScribeMD/slack-templates/.github/workflows/notify-assignee.yaml@0.6.3
+    permissions:
+      contents: read
     secrets:
       SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
       SLACK_ASSIGN_CHANNEL_ID: ${{ secrets.SLACK_ASSIGN_CHANNEL_ID }}

.github/workflows/notify-reviewers.yaml

@@ -7,6 +7,8 @@ jobs:
   notify-reviewers:
     name: Notify Reviewers
     uses: ScribeMD/slack-templates/.github/workflows/notify-reviewers.yaml@0.6.3
+    permissions:
+      contents: read
     secrets:
       SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
       SLACK_REVIEW_CHANNEL_ID: ${{ secrets.SLACK_REVIEW_CHANNEL_ID }}