Rebrand to INBUXA: product name, protocol greetings, sign-in page, calendar templates, logos, README

One branding module (types::brand!) supplies the name to every protocol
greeting, the HTTP realm, the startup banner, Received headers, the
user agent, the IMAP ID response and calendar PRODIDs. The sign-in and
calendar pages take ihasmail's palette and font and the INBUXA logo, and
calendar emails embed the INBUXA lockup. Default calendar and address book
names follow. Protocol identifiers (urn:stalwart:jmap, vnd.stalwart Sieve
extensions) and upstream copyright notices are unchanged.

crates/pop3/src/protocol/response.rs

@@ -114,7 +114,7 @@ impl<'x, T: Display> Response<'x, T> {
                     "EXPIRE NEVER",
                     "UIDL",
                     "UTF8",
-                    "IMPLEMENTATION Stalwart Server",
+                    concat!("IMPLEMENTATION ", types::brand_server!()),
                 ] {
                     buf.extend_from_slice(capa.as_bytes());
                     buf.extend_from_slice(b"\r\n");
@@ -200,7 +200,7 @@ mod tests {
                     "EXPIRE NEVER\r\n",
                     "UIDL\r\n",
                     "UTF8\r\n",
-                    "IMPLEMENTATION Stalwart Server\r\n.\r\n"
+                    concat!("IMPLEMENTATION ", types::brand_server!(), "\r\n.\r\n")
                 ),
             ),
             (