[SSR] Add Link Unfurling

We added react helmet in #603, but the og and other meta tags don't come through correctly in SSR.

In SSR, we are already injecting jsonLD into index.html here:

Lines 97 to 104 in 7bf4732

    
           try { 
        
             const headData = composeHeadData(dataset) 
        
             const jld = composeJSONLD(dataset) 
        
             indexHTML = indexHTML.replace('<head>', headData+jld) 
        
           } catch (e) { 
        
             console.log(`error composing dataset ${username/name} data into html tags: ${e}`) 
        
             return res.sendFile(indexPath) 
        
           }

We can use the same call to the preview to populate og tags and whatever else is necessary for link unfurling.

	try {
	const headData = composeHeadData(dataset)
	const jld = composeJSONLD(dataset)
	indexHTML = indexHTML.replace('<head>', headData+jld)
	} catch (e) {
	console.log(`error composing dataset ${username/name} data into html tags: ${e}`)
	return res.sendFile(indexPath)
	}