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).

package-lock.json

@@ -10,6 +10,7 @@
       "license": "ISC",
       "dependencies": {
         "@af-utils/scrollend-polyfill": "^0.0.14",
+        "@hrfee/simpletemplate": "^1.1.0",
         "@ts-stack/markdown": "^1.4.0",
         "@types/node": "^20.3.0",
         "@webcoder49/code-input": "^2.7.2",
@@ -482,6 +483,12 @@
         "node": ">=18"
       }
     },
+    "node_modules/@hrfee/simpletemplate": {
+      "version": "1.1.0",
+      "resolved": "https://registry.npmjs.org/@hrfee/simpletemplate/-/simpletemplate-1.1.0.tgz",
+      "integrity": "sha512-5H4/y7CegE4twstRPip/ms+OGUb+BPyVt+hriEpY88lTWW4I6jxzHFHmz6NDZmVyRa4RVIEVBxvtwk3RXKJVwA==",
+      "license": "MIT"
+    },
     "node_modules/@isaacs/cliui": {
       "version": "8.0.2",
       "resolved": "https://registry.npmjs.org/@isaacs/cliui/-/cliui-8.0.2.tgz",