discord: add/move to slash commands

the version of the discord library with support for this isn't
necessarily stable, so normal ! commands will still be available. The
user is no longer DMed for the PIN, instead they type /pin <PIN>.

views.go

@@ -504,7 +504,7 @@ func (app *appContext) InviteProxy(gc *gin.Context) {
 		data["discordUsername"] = app.discord.username
 		data["discordRequired"] = app.config.Section("discord").Key("required").MustBool(false)
 		data["discordSendPINMessage"] = template.HTML(app.storage.lang.Form[lang].Strings.template("sendPINDiscord", tmpl{
-			"command":        `<code class="code">` + app.config.Section("discord").Key("start_command").MustString("!start") + `</code>`,
+			"command":        `<code class="code">/` + app.config.Section("discord").Key("start_command").MustString("start") + `</code>`,
 			"server_channel": app.discord.serverChannelName,
 		}))
 		data["discordServerName"] = app.discord.serverName