Point the sign-in footer at ihasmail.org

The link went to linuxexpert.org, which is the organisation rather than
the project. Someone reading a sign-in page and following it wants the
software's own site, not its publisher's.

Checked that https://ihasmail.org answers 200 before putting it in front
of everyone who reaches the instance -- a dead link on a sign-in page is
worse than none.

The README badge still credits LINUXexpert.org, which is the right place
for attribution and was not part of this.

web/src/views/Login.tsx

@@ -101,7 +101,7 @@ export function LoginPage() {
             offer something a person can actually act on. It also means a bug
             report names the build without anyone having to sign in to find it.
           */}
-          ihasmail v{APP_VERSION} by <a href="https://linuxexpert.org" target="_blank" rel="noopener noreferrer">linuxexpert.org</a>
+          ihasmail v{APP_VERSION} by <a href="https://ihasmail.org" target="_blank" rel="noopener noreferrer">ihasmail.org</a>
           {" · "}
           <a href={sourceUrl} target="_blank" rel="noopener noreferrer">AGPL-3.0 source</a>
         </p>