problem: currency pages are not indexable

[AshHimself]

problem: currency pages are not indexable

Solution: make them indexable with some basic SEO.

[anbud]

@emurgobot claim

[anbud]

I actually found a better solution than the previous one. By using puppeteer we can render some SEO critical pages on the server and serve pure HTML to crawlers. Puppeteer is just a headless Chrome browser, so it renders pages perfectly fine.

I've implemented this approach on a couple of pages in #2072. Adding new pages is as easy as adding SeoRouter.route calls, but we shouldn't add too much of them (not all pages are SEO critical) as server rendering with puppeteer can be a little bit resource intensive. To kinda mitigate that, I've implemented a simple caching mechanism that stores generated HTML pages in a mongo collection. If it becomes a bottleneck, we can easily switch it to something more advanced, like Redis caching.

@emurgobot done