[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/attach.go

@@ -39,3 +39,10 @@ func (r *router) AttachMiddleware(middleware gin.HandlerFunc) {
 func (r *router) AttachNoRouteHandler(handler gin.HandlerFunc) {
 	r.engine.NoRoute(handler)
 }
+
+// AttachGroup attaches the given handlers into a group with the given relativePath as
+// base path for that group. It then returns the *gin.RouterGroup so that the caller
+// can add any extra middlewares etc specific to that group, as desired.
+func (r *router) AttachGroup(relativePath string, handlers ...gin.HandlerFunc) *gin.RouterGroup {
+	return r.engine.Group(relativePath, handlers...)
+}