Static fileserver improvements, optional admin panel route (#100)

* better asset serving, optional admin panel route

* linting

internal/web/fileserver.go

@@ -0,0 +1,47 @@
+/*
+   GoToSocial
+   Copyright (C) 2021 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
+}