invites: add /invites/send, store more details in new SentTo field

deprecated SendTo string field for SentTo, which holds successful send
addresses, and failures with a reason that isn't plain text. Will soon
add an interface for sending invites after their creation. For #444
(ha).

ts/modules/lang.ts

@@ -1,4 +1,5 @@
 import { _get } from "../modules/common.js";
+import { Template } from "@hrfee/simpletemplate";
 
 interface Meta {
     name: string;
@@ -39,6 +40,15 @@ export class lang implements Lang {
         return str;
     }
 
+    template = (sect: string, key: string, subs: { [key: string]: any }): string => {
+        if (sect == "quantityStrings" || sect == "meta") { return ""; }
+        const map = new Map<string, any>();
+        for (let key of Object.keys(subs)) { map.set(key, subs[key]); }
+        const [out, err] = Template(this._lang[sect][key], map);
+        if (err != null) throw err;
+        return out;
+    }
+
     quantity = (key: string, number: number): string => {
         if (number == 1) {
             return this._lang.quantityStrings[key].singular.replace("{n}", ""+number)