Relicense to the AGPL, version 3 or later

ihasmail is webmail: it is nearly always run as a network service rather
than handed to anyone as a binary, which is the case the plain GPL does
not reach. The AGPL's section 13 does — anyone running a modified
ihasmail for other people has to offer them its source.

LICENSE is the full AGPL-3.0 text from gnu.org. The SPDX identifier
changes from GPL-3.0-or-later to AGPL-3.0-or-later in package.json, the
lockfile's own entry for it, and the About screen. Old commits and tags
are left exactly as they were; this is the license from here on.

web/src/views/settings/AboutSettings.tsx

@@ -12,7 +12,7 @@ export function AboutSettings() {
         <img src="/img/logo.png" alt="ihasmail" width={96} />
         <div>
           <div style={{ fontWeight: 700, fontSize: "1.2em" }}>ihasmail 2.0</div>
-          <div className="hint">GPL-3.0-or-later · <a href="https://github.com/LINUXexpert-org/ihasmail" target="_blank" rel="noreferrer">github.com/LINUXexpert-org/ihasmail</a></div>
+          <div className="hint">AGPL-3.0-or-later · <a href="https://github.com/LINUXexpert-org/ihasmail" target="_blank" rel="noreferrer">github.com/LINUXexpert-org/ihasmail</a></div>
         </div>
       </div>
       <h2>Server</h2>