Migrate remaining files from the old version of the repo

- Migrate `CHANGELOG.md`, `.VERSION`, `LICENSE`, GitHub pipelines, release script, GitHub issue templates.
- Add in Makefile the commands to prepare release and make the release tag.

.github/workflows/release-pr.yml

@@ -0,0 +1,85 @@
+on:
+  create:
+    branches:
+
+name: Open Release PR for review
+
+jobs:
+  # This job is necessary because GitHub does not (yet) support
+  # filtering `create` triggers by branch name.
+  # See: https://github.community/t/trigger-job-on-branch-created/16878/5
+  should_create_pr:
+    name: Check if PR for branch already exists
+    runs-on: ubuntu-latest
+    outputs:
+      result: ${{ steps.is_release_branch_without_pr.outputs.result }}
+    steps:
+      - id: is_release_branch_without_pr
+        name: Find matching PR
+        uses: actions/github-script@v3
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            // Search for an existing PR with head & base
+            //  that match the created branch
+
+            const [releaseBranchName] = context.ref.match("release/v[0-9]+\.[0-9]+\.[0-9]+") || []
+
+            if(!releaseBranchName) { return false }
+
+            const {data: prs} = await github.pulls.list({
+                ...context.repo,
+                state: 'open',
+                head: `1Password:${releaseBranchName}`,
+                base: context.payload.master_branch
+            })
+
+            return prs.length === 0
+
+  create_pr:
+    needs: should_create_pr
+    if: needs.should_create_pr.outputs.result == 'true'
+    name: Create Release Pull Request
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Parse release version
+        id: get_version
+        run: echo "::set-output name=version::$(echo $GITHUB_REF | sed 's|^refs/heads/release/v?*||g')"
+
+      - name: Prepare Pull Request
+        id: prep_pr
+        run: |
+          CHANGELOG_PATH=$(printf "%s/CHANGELOG.md" "${GITHUB_WORKSPACE}")
+
+          LOG_ENTRY=$(awk '/START\/v[0-9]+\.[0-9]+\.[0-9]+*/{f=1; next} /---/{if (f == 1) exit} f' "${CHANGELOG_PATH}")
+          export PR_BODY=$(cat <<EOF
+          This is an automated PR for a new release.
+
+          Please check the following before approving:
+          - [ ] Changelog is accurate. The documented changes for this release are printed below.
+          - [ ] Any files referencing a version number. Confirm it matches the version number in the branch name.
+          ---
+          ## Release Changelog Preview
+          ${LOG_ENTRY}
+          EOF
+          )
+
+          # Sanitizes multiline strings for action outputs (https://medium.com/agorapulse-stories/23f56447d209)
+          PR_BODY="${PR_BODY//'%'/'%25'}"
+          PR_BODY="${PR_BODY//$'\n'/'%0A'}"
+          PR_BODY="${PR_BODY//$'\r'/'%0D'}"
+          echo "::set-output name=pr_body::$(echo "$PR_BODY")"
+
+      - name: Create Pull Request via API
+        id: post_pr
+        uses: octokit/request-action@v2.x
+        with:
+          route: POST /repos/${{ github.repository }}/pulls
+          title: ${{ format('Prepare Release - v{0}', steps.get_version.outputs.version) }}
+          head: ${{ github.ref }}
+          base: ${{ github.event.master_branch }}
+          body: ${{ toJson(steps.prep_pr.outputs.pr_body) }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}