Confirm before inviting unknown users to a DM/room (#33171)

* InviteDialog: factor out startDmOrSendInvites

Factor out the logic of calling `startDm` or `inviteUsers` to a helper
function. We're going to need to call this from a second location soon, so this
is useful groundwork.

* Add `UnknownIdentityUsersWarningDialog`

* Add unit tests

* Update playwright tests

* Convert if/else to switch statement

* Convert helper functions to React components

* Factor out "onRemove" callback

* Add clarifying comment

apps/web/playwright/e2e/room/create-room.spec.ts

@@ -57,6 +57,9 @@ test.describe("Create Room", () => {
 
         await page.getByRole("button", { name: "Go" }).click();
 
+        await expect(page.getByRole("heading", { name: "Start a chat with this new contact?" })).toBeVisible();
+        await page.getByRole("button", { name: "Continue" }).click();
+
         await expect(page.getByText("Encryption enabled")).toBeVisible();
         await expect(page.getByText("Send your first message to")).toBeVisible();