admin: add build tags to "About"

build tags like e2ee and external are now included in the about page.

internal.go

@@ -1,3 +1,4 @@
+//go:build !external
 // +build !external
 
 package main
@@ -10,6 +11,8 @@ import (
 
 const binaryType = "internal"
 
+func BuildTagsExternal() {}
+
 //go:embed data data/html data/web data/web/css data/web/js
 var loFS embed.FS