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-lock.json

@@ -7,7 +7,7 @@
     "": {
       "name": "ihasmail",
       "version": "2.0.0",
-      "license": "GPL-3.0-or-later",
+      "license": "AGPL-3.0-or-later",
       "workspaces": [
         "server",
         "web"