Add pull request templates for comon tasks

This is primarily aimed at preventing us from missing
any dependency review in the future.

.github/PULL_REQUEST_TEMPLATE/changelog_update.md

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+---
+name: Change log update
+about: Prepare the release of a Debian package.
+
+---
+
+## Description
+
+_Which package are you releasing? Please link to to **release tracking issue**._

.github/PULL_REQUEST_TEMPLATE/default.md

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+---
+name: Default
+about: For all the cases that don't have a specific template.
+
+---
+
+## Description
+
+_What is the PR doing, and why?_
+
+## Test plan
+
+- Automated tests ([Circle CI][ci]):
+  - [ ] All Bullseye jobs are passing
+  - Bookworm failures are not ignored:
+    - [ ] This PR does not introduce any new Bookworm failures
+    - [ ] An issue is open for all new Bookworm failures

.github/PULL_REQUEST_TEMPLATE/package_addition.md

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+---
+name: Package addition
+about: Add a dependency for one of our Debian packages.
+
+---
+
+## Description
+
+_Which packages are you adding, and why?_
+
+## Test plan
+
+- Automated tests ([Circle CI][ci]):
+  - [ ] All Bullseye jobs are passing
+  - [ ] Bookworm failures are not ignored:
+    - This PR does not introduce any new Bookworm failures
+    - An issue is open for all new Bookworm failures
+- Security:
+  - [ ] A [diff review][review-docs] was performed for all build dependencies
+
+  [ci]: https://app.circleci.com/pipelines/github/freedomofpress/securedrop-debian-packaging
+  [review-docs]: https://github.com/freedomofpress/securedrop/wiki/Dependency-specification-and-update-policies