Write the brand in lowercase where people see it

The name is inbuxa, lowercase, like the wordmark; INBUXA reads as an
acronym. The admin and webmail already changed. Here that's everything
the server shows people: the brand macro behind the protocol greetings,
the HTTP and SCIM realms, the startup banner and the calendar and contact
PRODID; the first-party OAuth client descriptions; the legacy-protocol
refusals; the default calendar and address book names and the SMTP
greeting default, in the code and the schema served to the admin
(checksum regenerated); startup and shutdown events; the User-Agent;
the sign-in and RSVP pages; the service units; the OpenAPI realm; the
crate descriptions and the README, where it's set in bold.

Identifiers that are uppercase for their own reasons stay: INBUXA_*
settings, SUBSPACE_INBUXA. So do code comments and the AGPL 5(a) notice
lines.

Tests follow: the IMAP ID name, the default collection names, the PRODID
in the iTIP fixtures and the CalDAV free-busy expectations, and the e2e
legacy-protocol refusals. The webdav, imap and jmap suites pass, so do
the unit tests of every crate touched, and 73 of 75 SMTP tests; of the
other two, antispam fails on main too, and queue_retry is a timing flake
that passes on its own.

tests/e2e/legacy_protocols.py

@@ -49,7 +49,7 @@ HTTP = "http://127.0.0.1:18080"
 # made to speak.
 PORTS = {"imap": 18993, "pop3": 18995, "submissions": 18465, "smtp": 18025}
 TLS_PORTS = {18993, 18995, 18465}
-SMTP_REFUSAL = ("535 5.7.0 This server allows only INBUXA webmail and JMAP apps. "
+SMTP_REFUSAL = ("535 5.7.0 This server allows only inbuxa webmail and JMAP apps. "
                 "This mail app can't send.")
 INBUXA = "urn:inbuxa:jmap"
 failures = []
@@ -333,7 +333,7 @@ def tenant_checks(admin, admin_pw, account):
           "and still enabled for an account outside the tenant (test 13)")
 
     # Refused on the tenant's domain, every way in the same words (tests 6-8).
-    imap_no = ("NO [ALERT] Your organization allows only INBUXA webmail and JMAP apps. "
+    imap_no = ("NO [ALERT] Your organization allows only inbuxa webmail and JMAP apps. "
                "This mail app can't sign in.")
     check(imap_login(PORTS["imap"], tu, user_pw) == imap_no,
           "the tenant's user is refused over IMAP with the right password (test 6)")
@@ -341,10 +341,10 @@ def tenant_checks(admin, admin_pw, account):
     check(imap_login(PORTS["imap"], "[email protected]", "x") == imap_no,
           "and a made-up address on the domain gets the same (test 7)")
     check(pop3_login(PORTS["pop3"], tu, user_pw) ==
-          "-ERR [AUTH] Your organization allows only INBUXA webmail and JMAP apps. "
+          "-ERR [AUTH] Your organization allows only inbuxa webmail and JMAP apps. "
           "This mail app can't sign in.", "POP3 refuses in its own form (test 8)")
     check(smtp_auths(PORTS["submissions"], tu, [user_pw])[0] ==
-          "535 5.7.0 Your organization allows only INBUXA webmail and JMAP apps. "
+          "535 5.7.0 Your organization allows only inbuxa webmail and JMAP apps. "
           "This mail app can't send.", "submission refuses in its own form (test 8)")
     check(imap_login(PORTS["imap"], admin, admin_pw).startswith("OK"),
           "an account on another domain signs in over IMAP normally (test 6)")