[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/web/fileserver.go

@@ -1,47 +0,0 @@
-/*
-   GoToSocial
-   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
-
-   This program is free software: you can redistribute it and/or modify
-   it under the terms of the GNU Affero General Public License as published by
-   the Free Software Foundation, either version 3 of the License, or
-   (at your option) any later version.
-
-   This program is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-   GNU Affero General Public License for more details.
-
-   You should have received a copy of the GNU Affero General Public License
-   along with this program.  If not, see <http://www.gnu.org/licenses/>.
-*/
-
-package web
-
-import (
-	"net/http"
-	"strings"
-)
-
-type fileSystem struct {
-	fs http.FileSystem
-}
-
-// FileSystem server that only accepts directory listings when an index.html is available
-// from https://gist.github.com/hauxe/f2ea1901216177ccf9550a1b8bd59178
-func (fs fileSystem) Open(path string) (http.File, error) {
-	f, err := fs.fs.Open(path)
-	if err != nil {
-		return nil, err
-	}
-
-	s, _ := f.Stat()
-	if s.IsDir() {
-		index := strings.TrimSuffix(path, "/") + "/index.html"
-		if _, err := fs.fs.Open(index); err != nil {
-			return nil, err
-		}
-	}
-
-	return f, nil
-}