Mint a unique identifier for each PWA and use it to link to PWAStats.com

[adewale]

1. Mint an identifier for each PWA based on the rules that come out of the discussion here: Add a unique identifier for a PWA w3c/manifest#586 and here: Manifest processing should not be a function of document URL w3c/manifest#668
2. For each PWA link (using rel=nofollow to avoid any potential SEO issues) to: pwastats.com This means if there are no stats for a given PWA the link will 404 but that's tolerable in the short term since if any of these PWAs ever have stats the link will start working.
3. On pwastats.com for each PWA that has stats link (using rel=nofollow) to pwadirectory.appspot.com
4. Done properly a PWA such as treebo would have: https://www.pwastats.com/2018/04/treebo/ linking bidirectionally to: https://pwa-directory.appspot.com/pwas/5187301748506624

In terms of implementation we'll need some mechanism that lets a site like PWA Directory redirect a link to https://pwa-directory.appspot.com/id/$NORMALISED_STANDARD_IDENTIFIER into a link to: https://pwa-directory.appspot.com/pwas/5187301748506624 We'd then need to sufficiently document the normalisation function (since it's looking like we're going to converge on manifest URL as the standard identifier for PWAs) so that pwastats.com and other sites can implement it.

[adewale]

/cc @andreban @juliantoledo @ithinkihaveacat @grigs