Final review

- grammatical fixes
- normalization of video display
- formatting (to help maintenance later)
- minor CSS tweaks
Release tools

Automate releases on the website.

- Provided mechanism for retrieving release data from GitHub
  (`bin/get-releases.php`).
- Provided class for parsing GitHub release data and aggregating tag, URI, and
  changelog content (`Application\GithubReleases`).
- Modified `DownloadController` to accept a `GithubReleases` instance, and to
  use it to create the list of releases, and inject a changelog into the notes
  view.
- Updated the download changelog notes view to parse the Markdown-formatted
  changelogs into HTML.
- Created a `config/autoload/global.php.dist` file that substitutes a templated
  string for versions.
- Created a Makefile that:
    - calls `sed` to substitute the provided version string into
      `config/autoload/global.php.dist` and write it to
      `config/autoload/global.php`.
    - invokes `bin/get-releases.php`.