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.

web/src/views/admin/__tests__/admin-dashboard.test.tsx

@@ -112,7 +112,7 @@ describe("the Administration dashboard", () => {
   });
 });
 
-describe("the pointer to Stalwart's own administration", () => {
+describe("the pointer to INBUXA Admin", () => {
   let host: HTMLDivElement;
   let root: Root;
   beforeEach(() => {
@@ -136,7 +136,7 @@ describe("the pointer to Stalwart's own administration", () => {
   it("names it, and links it where the operator has said where it is", async () => {
     await renderWith("https://admin.example.com");
     const note = host.querySelector(".admin-dashboard-note")!;
-    expect(note.textContent).toContain("Stalwart's own administration");
+    expect(note.textContent).toContain("INBUXA Admin");
     const link = note.querySelector("a")!;
     expect(link.getAttribute("href")).toBe("https://admin.example.com");
     expect(link.getAttribute("rel")).toBe("noopener noreferrer");
@@ -144,7 +144,7 @@ describe("the pointer to Stalwart's own administration", () => {
 
   it("names it without a link where nobody has", async () => {
     await renderWith(null);
-    expect(host.querySelector(".admin-dashboard-note")?.textContent).toContain("Stalwart's own administration");
+    expect(host.querySelector(".admin-dashboard-note")?.textContent).toContain("INBUXA Admin");
     expect(host.querySelector(".admin-dashboard-note a")).toBeNull();
   });
 });