[feature] Implement cache-control and etags for static assets (#711)

* start working on etag stuff

* add + use cache middleware

* generate etags on the fly

* remove unused field

* clean up filepath

* add license headers to cache files

* add attachgroup function to router interface

* move cache into web module

* rename a couple things

* remove attachStaticFS function from router

* rename + tidy up a few things

* mount assets filesystem

* create assetsFileInfoCache

* update comment

* simplify hash

* fix string fmt

* skip last mod chk, prefer strong etags w/long cache

* move base handler to its own file
this matches the modules in the api folder

* generate new etag if file was modified

* wrap strong etag in quotation marks as per spec

* clarify logic in avatar search

* make hashing a little niftier

internal/router/router.go

@@ -47,8 +47,10 @@ type Router interface {
 	AttachMiddleware(handler gin.HandlerFunc)
 	// Attach 404 NoRoute handler
 	AttachNoRouteHandler(handler gin.HandlerFunc)
-	// Add Gin StaticFS handler
-	AttachStaticFS(relativePath string, fs http.FileSystem)
+	// Attach a router group, and receive that group back.
+	// More middlewares and handlers can then be attached on
+	// the group by the caller.
+	AttachGroup(path string, handlers ...gin.HandlerFunc) *gin.RouterGroup
 	// Start the router
 	Start()
 	// Stop the router
@@ -62,11 +64,6 @@ type router struct {
 	certManager *autocert.Manager
 }
 
-// Add Gin StaticFS handler
-func (r *router) AttachStaticFS(relativePath string, fs http.FileSystem) {
-	r.engine.StaticFS(relativePath, fs)
-}
-
 // Start starts the router nicely. It will serve two handlers if letsencrypt is enabled, and only the web/API handler if letsencrypt is not enabled.
 func (r *router) Start() {
 	// listen is the server start function, by