user_groups: Add get_recursive_group_members.

This function goes through all subgroups recursively and returns the
resultant set of the members of those subgroups in addition to the
members of our target_group.

This function is required in order to add the `everyone` pill to create
channel flow.

For the tests written in this commit, it uses the same pattern as the
`get_recursive_subgroups` test.

`is_user_in_group` could have been technically refactored to use
`get_recursive_group_members`, but since the former returns early for
direct members, I've let it be for now.

web/src/user_groups.ts

@@ -1,3 +1,4 @@
+import assert from "minimalistic-assert";
 import type {z} from "zod";
 
 import * as blueslip from "./blueslip";
@@ -204,6 +205,24 @@ export function get_recursive_subgroups(target_user_group: UserGroup): Set<numbe
     return subgroup_ids;
 }
 
+export function get_recursive_group_members(target_user_group: UserGroup): Set<number> {
+    const members = new Set(target_user_group.members);
+    const subgroup_ids = get_recursive_subgroups(target_user_group);
+
+    if (subgroup_ids === undefined) {
+        return members;
+    }
+
+    for (const subgroup_id of subgroup_ids) {
+        const subgroup = user_group_by_id_dict.get(subgroup_id);
+        assert(subgroup !== undefined);
+        for (const member of subgroup.members) {
+            members.add(member);
+        }
+    }
+    return members;
+}
+
 export function is_user_in_group(user_group_id: number, user_id: number): boolean {
     const user_group = user_group_by_id_dict.get(user_group_id);
     if (user_group === undefined) {