messages: custom content described in customcontent.go, message tests

customcontent.go constains a structure with all the custom content,
methods for getting display names, subjects, etc., and a list of
variables, conditionals, and placeholder values. Tests for constructX
methods included in email_test.go, and all jfa-go tests can be run with
make INTERNAL=off test.

external.go

@@ -4,7 +4,6 @@
 package main
 
 import (
-	"io/fs"
 	"log"
 	"os"
 	"path/filepath"
@@ -15,9 +14,6 @@ const binaryType = "external"
 
 func BuildTagsExternal() { buildTags = append(buildTags, "external") }
 
-var localFS dirFS
-var langFS dirFS
-
 // When using os.DirFS, even on Windows the separator seems to be '/'.
 // func FSJoin(elem ...string) string { return filepath.Join(elem...) }
 func FSJoin(elem ...string) string {
@@ -32,20 +28,6 @@ func FSJoin(elem ...string) string {
 	return strings.TrimSuffix(path, sep)
 }
 
-type dirFS string
-
-func (dir dirFS) Open(name string) (fs.File, error) {
-	return os.Open(string(dir) + "/" + name)
-}
-
-func (dir dirFS) ReadFile(name string) ([]byte, error) {
-	return os.ReadFile(string(dir) + "/" + name)
-}
-
-func (dir dirFS) ReadDir(name string) ([]fs.DirEntry, error) {
-	return os.ReadDir(string(dir) + "/" + name)
-}
-
 func loadFilesystems() {
 	log.Println("Using external storage")
 	executable, _ := os.Executable()