Lazy load the scripts needed for certain sources

[rjyo]

Problem: When not all the pages need embedding, using SSR to add the scripts (on all pages) will damage the whole site's speed score (benchmarked with Google's PageSpeed Insights).

Solution: This PR will move the SSR injected scripts to the client side. As thus they will only be loaded on the pages(blogs) with embeds.

[raae]

I have been thinking it should be possible to check for the embeds server-side as well. Add a flag to the pages when embeds are processed.

I am a little afraid to check for things like blockquote.instagram-media as they might change the html we get back from their endpoints at any time.

[raae]

We might secretly add the elements to options, so if they break user can fix it by overriding the elements without having to wait for us to fix it.

[nickytonline]

Just following up on this PR. I like the idea of this as we should only be loading scripts that we require. Is this something you still want to work on @rjyo?