GitHub Actions: Automatically fill in blog post date after merge

[Holzhaus]

After merging a new blog post into the website branch, this automatically updates the blog post date to the commit date of the merge commit if the filename starts with XXXX-XX-XX- or 2020-XX-XX. It renames the file and updates the date: entry, then commits and pushes the changes automatically.

Afterwards, it automatically triggers a redeploy on Netlify if the NETLIFY_BUILD_HOOK secret is defined: https://docs.netlify.com/configure-builds/build-hooks/
(git pushes from GitHub don't trigger web hooks automatically to prevent endless loops)

Therefore, we don't have to keep up with bumping the dates manually all the time.

[Holzhaus]

If you want to test this on your fork:

Merge this branch into the website branch, then push the website branch to your fork. If you have a Netlify page for your fork, generate a build hook URL and add it to your repository's secrets (Repository Settings -> Secrets).

Then open a Pull Request against the website branch of your fork that adds a new blog post with filename XXXX-XX-XX-somename.html or 2020-XX-XX-somename.html. For example, you can just use my serato blogpost branch: #81

Then merge the PR on your fork. Github actions should automatically update the date, commit and push. If you've confogured the NETLIFY_BUILD_HOOK secret, a Netlify deploy will be triggered automatically.

[Holzhaus]

@daschuer Please add the build hook secret to this repo.

[Holzhaus]

Merge? Then I can update #167 with the necessary changes.