Prerender static pages in post_deploy hook

[nykula]

Re: #417, #423

Introduce an app:prerender command that renders home page, Download, News, Get Involved, Showcase, Competitions, Branding and Wiki to static pages in public/. Call it from hooks/post_deploy - is this how the website gets updated in production? You can also call it in cron.

Major difference: trailing slashes, because it's easiest to get working. Where there was /download, now there is /download/. The web server should redirect to a trailing slash automatically when it sees a directory.

Change e.g. /download/artifact/... to /download-artifact/... so that the /download/ static page doesn't prevent nightly downloads from working. Well, they still don't work while the backend is down due to high load, but I get 403 errors from GitHub API when trying to generate multiple download URLs in a row for static pages, so leaving this as is for now.

Add an http-equiv redirect under /documentation/. Not a good way to make a redirect, but better than a 502 error.

Note: I think one will need to change web server config so that the HTML is cached at most for a day.

Would these changes be okay?

[nykula]

Added code that applies translations client-side. It relies on the following being run somewhere on server (if not run, nothing breaks, the website just remains in English):

tx pull -a --minimum-perc 85 --json
mv translations/*json public/translations/

For tx pull, an environment variable TX_TOKEN needs to be set to a token generated in Transifex. The token gives access to the Transifex account API so should be carefully kept as a backend secret.