[auspice view] improve cache-control settings

This commit (building on top of #1126) enforces that
index.html is never cached, as it gets recreated when auspice rebuilds and the src for the main JS bundle changes accordingly. This may not be strictly necessary, as the max-age was 0 and the ETag changes as the file changes, but setting this explicitly is good practice.

JS bundles, which use hashes to facilitate cache busting, are served with a 30d cache lifetime. Note that this is not enough to _force_ browsers to not revalidate within the 30d (i.e. 304 response) -- see [this post](https://stackoverflow.com/questions/26166433/how-to-prevent-request-that-returns-304/26339940#26339940) for context. Revalidation during the 30d lifetime will return a 304 which is no worse than the current situation we have.

cli/view.js

@@ -113,7 +113,7 @@ const run = (args) => {
   utils.verbose(`Serving index / favicon etc from  "${auspiceBuild.baseDir}"`);
   utils.verbose(`Serving built javascript from     "${auspiceBuild.distDir}"`);
   app.get("/favicon.png", (req, res) => {res.sendFile(path.join(auspiceBuild.baseDir, "favicon.png"));});
-  app.use("/dist", expressStaticGzip(auspiceBuild.distDir));
+  app.use("/dist", expressStaticGzip(auspiceBuild.distDir, {maxAge: '30d'}));
 
   let handlerMsg = "";
   if (args.gh_pages) {
@@ -124,7 +124,7 @@ const run = (args) => {
 
   /* this must be the last "get" handler, else the "*" swallows all other requests */
   app.get("*", (req, res) => {
-    res.sendFile(path.join(auspiceBuild.baseDir, "dist/index.html"));
+    res.sendFile(path.join(auspiceBuild.baseDir, "dist/index.html"), {headers: {"Cache-Control": "no-cache, no-store, must-revalidate"}});
   });
 
   const server = app.listen(app.get('port'), app.get('host'), () => {