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.

package.json

@@ -3,7 +3,7 @@
   "version": "2.0.0",
   "private": true,
   "description": "ihasmail \u2014 a fast, modern JMAP webmail for Stalwart Mail Server",
-  "license": "GPL-3.0-or-later",
+  "license": "AGPL-3.0-or-later",
   "type": "module",
   "workspaces": [
     "server",