[keyserver] Use blank target in invite link

Summary:
This page is displayed only on iOS devices. For Safari, this changes invite links experience a bit - the browser shows an alert

{F630049}

asking if a page should be opened instead of showing a banner.

{F630050}

It is also possible that this diff could solve an issue on other browsers https://linear.app/comm/issue/ENG-4089/invite-links-dont-work-on-prod#comment-2e1dff05.

Also included a small change where `inviteLinkUrl` is used instead of hardcoded URL.

Test Plan: Open a link and check if the app is opened (both on Safari and Chrome).

Reviewers: kamil, inka, bartek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8465

keyserver/src/responders/website-responders.js

@@ -12,6 +12,7 @@ import ReactDOMServer from 'react-dom/server';
 import t from 'tcomb';
 import { promisify } from 'util';
 
+import { inviteLinkUrl } from 'lib/facts/links.js';
 import { baseLegalPolicies } from 'lib/facts/policies.js';
 import stores from 'lib/facts/stores.js';
 import { daysToEntriesFromEntryInfos } from 'lib/reducers/entry-reducer.js';
@@ -791,7 +792,8 @@ async function inviteResponder(req: $Request, res: $Response): Promise<void> {
             <a class="button" href="${stores.appStoreUrl}">Download Comm</a>
             <a
               class="button secondary"
-              href="https://comm.app/invite/${secret}"
+              href="${inviteLinkUrl(secret)}"
+              target="_blank"
             >
               Invite Link
             </a>