add announcement emails

After selecting users in the accounts tab, you can press 'Announce',
then write a subject and message (with markdown), and an email will be
sent to each selected user.

config.go

@@ -63,6 +63,9 @@ func (app *appContext) loadConfig() error {
 	app.config.Section("welcome_email").Key("email_html").SetValue(app.config.Section("welcome_email").Key("email_html").MustString("jfa-go:" + "welcome.html"))
 	app.config.Section("welcome_email").Key("email_text").SetValue(app.config.Section("welcome_email").Key("email_text").MustString("jfa-go:" + "welcome.txt"))
 
+	app.config.Section("announcement_email").Key("email_html").SetValue(app.config.Section("announcement_email").Key("email_html").MustString("jfa-go:" + "announcement.html"))
+	app.config.Section("announcement_email").Key("email_text").SetValue(app.config.Section("announcement_email").Key("email_text").MustString("jfa-go:" + "announcement.txt"))
+
 	app.config.Section("jellyfin").Key("version").SetValue(VERSION)
 	app.config.Section("jellyfin").Key("device").SetValue("jfa-go")
 	app.config.Section("jellyfin").Key("device_id").SetValue(fmt.Sprintf("jfa-go-%s-%s", VERSION, COMMIT))