AGPL source offer and name cleanup

Every build writes the exact source it was built from, uncommitted work and
new files included, as source.tar.gz next to the app, named after that tree.
Docker builds, which have no git, pack the build context and name it by a
hash of its files. The sign-in page and Settings > About link to it instead of
a repository that can drift.

What users, operators and packagers see no longer names the upstream server:
- interface text, in all nine catalogues, with a token-session line for
  Security;
- server messages;
- the settings, now MAIL_SERVER_URL, MAIL_SERVERS_FILE, ADMIN_URL and
  MAIL_SERVER_FOLLOW_ADVERTISED_URLS, and mail-servers.example.json;
- the Tenants notice, which is gone;
- the README, CONTRIBUTING and SECURITY.

ihasmail's own FEATURES, KNOWN-ISSUES and ROADMAP stay with public ihasmail,
and INBUXA.md is folded into the README.

server/src/login-guard.test.ts

@@ -19,7 +19,7 @@ process.env.MOCK_PORT = String(PORT);
 process.env.MOCK_USER = "[email protected]";
 process.env.MOCK_PASS = "demo-password";
 process.env.MOCK_NO_REGISTRY = "1"; // a server without urn:stalwart:jmap
-process.env.STALWART_URL = `http://127.0.0.1:${PORT}`;
+process.env.MAIL_SERVER_URL = `http://127.0.0.1:${PORT}`;
 process.env.APP_SECRET = "test-secret-for-login-guard";
 
 const mock = await import("./mock/index.js");
@@ -48,13 +48,12 @@ test("a server without the registry is refused, with good credentials", async ()
   assert.equal(res.body.error, "unsupported_server");
 });
 
-test("the message says the credentials were fine, and names the way out", async () => {
+test("the message says the credentials were fine, and that the server isn't supported", async () => {
   const { body } = await login({ username: "[email protected]", password: "demo-password" });
   // Someone hitting this has typed a correct password. Saying so is the
-  // difference between "upgrade your server" and "try your password again".
+  // difference between "wrong server" and "try your password again".
   assert.match(body.message, /credentials are fine/i);
-  assert.match(body.message, /0\.16/);
-  assert.match(body.message, /stalwart-0\.15-support/, "the tag to build from if they cannot upgrade");
+  assert.match(body.message, /isn't one this webmail supports/);
 });
 
 test("no session is minted for a server we cannot talk to", async () => {