Monstache middleware in golang

[skzap]

The search engine is struggling since because of the 'tags' object in the contents collection. Elastic search creates an index for each new tag used on DTube, and it's not a good practise for elastic search data (increased ram usage for no reason).

So we need a monstache middleware to convert the tags object into a normal array, before it gets added to elastic search. Then some changes in UI will be required too to make the tags navigation work again.