Add workflow to check signoffs on PRs (#818)

* Add signoff workflow

* Add bridges

* changelog

.github/CODEOWNERS

+* @matrix-org/bridges
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.github/workflows/sign-off.yml

+name: Contribution requirements
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize]
+
+jobs:
+  signoff:
+    uses: matrix-org/backend-meta/.github/workflows/sign-off.yml@v1.4.0
+          

changelog.d/818.misc

+Add new CI workflow to check for signoffs.
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