New release workflow triggered by the merge of a PR changing

IS_DEVELOPMENT_BUILD to false

This is more streamlined than the previous approach, and works better
for a world where 1 person isn't doing all the work.

Now, the flow is simpler:
- Do changes (e.g. protocol update), changelog & set IS_DEVELOPMENT_BUILD to false all in a single PR, which can be squash-merged if desired
- Once the PR is merged, a draft release will be prepared
- RestrictedActions will automatically set IS_DEVELOPMENT_BUILD back to
  true and bump the version
- Tag will be created when the release is published

Previously, multiple PRs might be needed, and the PR containing the
release changelog couldn't be squash-merged. Manual intervention was
also required to create a tag and prepare a release.

This PR also includes new CI checks to check for basic errors like
forgotten changelog files to ensure changelog links work correctly.

Note: Only PRs from PMMP Team members with **write** access to the
repository can trigger release generation. Random people cannot trigger
release generation by sending PRs.

.github/workflows/draft-release-from-pr.yml

@@ -0,0 +1,65 @@
+name: Draft release from PR
+
+on:
+  #presume that pull_request_target is safe at this point, since the PR was approved and merged
+  #we need write access to prepare the release & create comments
+  pull_request_target:
+    types:
+      - closed
+    branches:
+      - stable
+      - minor-next
+      - major-next
+      - "legacy/*"
+    paths:
+      - "src/VersionInfo.php"
+
+jobs:
+  check:
+    name: Check release
+    uses: ./.github/workflows/draft-release-pr-check.yml
+
+  draft:
+    name: Create GitHub draft release
+    needs: [check]
+    if: needs.check.outputs.valid == 'true'
+
+    uses: ./.github/workflows/draft-release.yml
+
+  post-draft-url-comment:
+    name: Post draft release URL as comment
+    needs: [draft]
+
+    runs-on: ubuntu-20.04
+
+    steps:
+      - name: Post draft release URL on PR
+        uses: thollander/actions-comment-pull-request@v2
+        with:
+          message: "[Draft release ${{ needs.draft.outputs.version }}](${{ needs.draft.outputs.draft-url }}) has been created for commit ${{ github.sha }}. Please review and publish it."
+
+  trigger-post-release-workflow:
+    name: Trigger post-release RestrictedActions workflow
+    # Not sure if needs is actually needed here
+    needs: [check]
+    if: needs.check.outputs.valid == 'true'
+
+    runs-on: ubuntu-20.04
+
+    steps:
+      - name: Generate access token
+        id: generate-token
+        uses: actions/create-github-app-token@v1
+        with:
+          app-id: ${{ vars.RESTRICTED_ACTIONS_DISPATCH_ID }}
+          private-key: ${{ secrets.RESTRICTED_ACTIONS_DISPATCH_KEY }}
+          owner: ${{ github.repository_owner }}
+          repositories: RestrictedActions
+
+      - name: Dispatch post-release restricted action
+        uses: peter-evans/repository-dispatch@v3
+        with:
+          token: ${{ steps.generate-token.outputs.token }}
+          repository: ${{ github.repository_owner }}/RestrictedActions
+          event-type: pocketmine_mp_post_release
+          client-payload: '{"branch": "${{ github.ref }}"}'