Enable caching of static assets

[AlexeySachkov]

This PR solves one of items described in #1087: it enables cache for static assets.

The cache-control is different for various assets: .js and .css files are allowed to be cached by client, but only up to 12 hours and after that client must revalidate them to see if there were any updates; images and fonts are cached up to a months

[calculuschild]

Nice. By any chance is there a way to do this without adding the Mime package? It's pretty small, but just wanted to be sure.

[AlexeySachkov]

Nice. By any chance is there a way to do this without adding the Mime package? It's pretty small, but just wanted to be sure.

mime is actually already included into our dependencies transitively, the chain is express-static-gzip <- serve-static <- send <- mime, but the version used there is old and not user-friendly so I explicitly added it one more time.

I think it is possible to just check for file extensions by ourselves: basically we only need to distinguish between .css, .js and the rest at this point. Moreover, since we use compression I anyway had to add a check for .br suffix. Let me prepare a second version to see if it would be better

[calculuschild]

Ok. Well if we already have it as a deeper dependency that's less of an issue, but I want to keep things clean as we go if we can. I was mostly curious if Javascript/etc. already had a native function to do that.

I just skimmed over Mime.js and from what I could tell it is literally just looking up mime types out of a table based on file extension anyway.

Mime.prototype.getType = function(path) {
    path = String(path);
    let last = path.replace(/^.*[/\\]/, '').toLowerCase();
    let ext = last.replace(/^.*\./, '').toLowerCase();
    let hasPath = last.length < path.length;
    let hasDot = ext.length < last.length - 1;
    return (hasDot || !hasPath) && this._types[ext] || null;
}; //this._types is just a huge array

[AlexeySachkov]

@calculuschild, I've removed dependency on mime in fa20618

[calculuschild]

Cool. I'll be a couple of days before I can review and merge this (job deadlines) but this should go through easily enough.