DEV: Publish and enable use of pre-built JS assets (#33973)

Building the Discourse ember app is resource-intensive process. This
commit introduces a framework for us to build these assets centrally,
and make them available for people to download.

On every commit to `main`, a new GitHub actions workflow will build
development & production versions of the core assets, and publish them
as a github release under the `discourse/discourse-assets` repository. A
separate repository is being used to avoid polluting the main
`discourse/discourse` repository with one-git-tag-per-release.

The `assemble_ember_build.rb` script is updated to fetch the relevant
asset bundle. Requests are made to `get.discourse.org`, which then
redirects to GitHub releases. This redirection service is being used so
that we have the option to switch away from GitHub releases in future
without breaking existing Discourse installations.

For now, this behavior can be enabled by setting
`DISCOURSE_DOWNLOAD_PRE_BUILT_ASSETS=1`. In the near future, we hope to
make this the default, with opt-out via
`DISCOURSE_DOWNLOAD_PRE_BUILT_ASSETS=0`.

.github/workflows/publish-assets.yml

@@ -0,0 +1,52 @@
+name: Publish Assets
+
+on:
+  push:
+    branches:
+      - main
+      - stable
+
+concurrency:
+  group: publish-assets-${{ format('{0}-{1}', github.ref, github.job) }}
+  cancel-in-progress: true
+
+jobs:
+  publish-assets:
+    if: github.repository == 'discourse/discourse'
+    runs-on: 'debian-12-8core'
+    container: discourse/discourse_test:release
+    timeout-minutes: 30
+  
+    steps:
+      - name: Install gh cli
+        run: |
+          # https://github.com/cli/cli/blob/trunk/docs/install_linux.md#debian-ubuntu-linux-raspberry-pi-os-apt
+          (type -p wget >/dev/null || (sudo apt update && sudo apt install wget -y)) \
+            && sudo mkdir -p -m 755 /etc/apt/keyrings \
+            && out=$(mktemp) && wget -nv -O$out https://cli.github.com/packages/githubcli-archive-keyring.gpg \
+            && cat $out | sudo tee /etc/apt/keyrings/githubcli-archive-keyring.gpg > /dev/null \
+            && sudo chmod go+r /etc/apt/keyrings/githubcli-archive-keyring.gpg \
+            && sudo mkdir -p -m 755 /etc/apt/sources.list.d \
+            && echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/githubcli-archive-keyring.gpg] https://cli.github.com/packages stable main" | sudo tee /etc/apt/sources.list.d/github-cli.list > /dev/null \
+            && sudo apt update \
+            && sudo apt install gh -y
+      - name: Set working directory owner
+        run: chown root:root .
+
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 1
+
+      - name: Setup Git
+        run: |
+          git config --global user.email "ci@ci.invalid"
+          git config --global user.name "Discourse CI"
+
+      - name: pnpm install
+        run: pnpm install --frozen-lockfile
+
+      - name: build and release assets
+        env:
+          GITHUB_TOKEN: ${{ secrets.DISCOURSE_ASSETS_RELEASE_TOKEN }}
+        run: |
+          script/publish_built_assets.rb