Set default `og:ttl` tag

[owaincuvelier]

The facebook crawler can be quite aggressive. Most sites probably want this traffic, so outright blocking the user agent or limiting it via WAF may not be a desireable result. As such, we can implement a (I believe meta tag) tag as mentioned here: https://stackoverflow.com/questions/11521798/excessive-traffic-from-facebookexternalhit-bot