invite: Add frontend support to send an invite which never expires.

Fixes part of #20337.

static/js/invite.js

@@ -34,7 +34,15 @@ function reset_error_messages() {
 
 function get_common_invitation_data() {
     const invite_as = Number.parseInt($("#invite_as").val(), 10);
-    const expires_in = Number.parseFloat($("#expires_in").val());
+    let expires_in = $("#expires_in").val();
+
+    // See settings_config.expires_in_values for why we do this conversion.
+    if (expires_in === "null") {
+        expires_in = JSON.stringify(null);
+    } else {
+        expires_in = Number.parseFloat($("#expires_in").val());
+    }
+
     const stream_ids = [];
     $("#invite-stream-checkboxes input:checked").each(function () {
         const stream_id = Number.parseInt($(this).val(), 10);

static/js/settings_config.ts

@@ -467,12 +467,15 @@ export const expires_in_values = {
         description: $t({defaultMessage: "30 days"}),
         default: false,
     },
-    // Backend support for this configuration is not available yet.
-    // never: {
-    //     value: "never",
-    //     description: $t({defaultMessage: "Never expires"}),
-    //     default: false,
-    // }
+    never: {
+        // Ideally we'd just store `null`, not the string `"null"`, but
+        // .val() will read null back as `""`.  Custom logic in
+        // get_common_invitation_data converts this back to `null`
+        // before sending to the server.
+        value: "null",
+        description: $t({defaultMessage: "Never expires"}),
+        default: false,
+    },
 };
 
 const user_role_array = Object.values(user_role_values);

static/js/settings_invites.js

@@ -62,7 +62,9 @@ function populate_invites(invites_data) {
         name: "admin_invites_list",
         modifier(item) {
             item.invited_absolute_time = timerender.absolute_time(item.invited * 1000);
+            if (item.expiry_date !== null) {
                 item.expiry_date_absolute_time = timerender.absolute_time(item.expiry_date * 1000);
+            }
             item.is_admin = page_params.is_admin;
             item.disable_buttons =
                 item.invited_as === settings_config.user_role_values.owner.code &&

static/templates/settings/admin_invites_list.hbs

@@ -22,7 +22,11 @@
         <span class="invited_at">{{invited_absolute_time}}</span>
     </td>
     <td>
+        {{#if expiry_date_absolute_time}}
         <span class="expires_at">{{expiry_date_absolute_time}}</span>
+        {{else}}
+        <span class="expires_at">{{t "Never expires"}}</span>
+        {{/if}}
     </td>
     <td>
         <span>{{invited_as_text}}</span>