Take a token, never a password, outside DAV

Anyone could host a copy of a front end on a server of their own,
collect a person's password there, and replay it as HTTP Basic against
JMAP or the API. Cross-origin rules don't stop that, since a server
isn't a browser, and neither does client registration, since Basic
never goes through OAuth (contract C-23).

JMAP (session, API, upload, download, event source, WebSocket), /api,
/auth/introspect, /auth/userinfo and authenticated /auth/register now
refuse an Authorization: Basic header before looking at the password,
with a 401 whose only challenge is Bearer. A wrong password gets the
same answer as the right one. CalDAV and CardDAV keep Basic, and their
401s still offer it. The sign-in page's /api/auth takes the password in
its body and is unaffected, as is the token endpoint's client
authentication.

Bootstrap and recovery mode accept Basic everywhere, as they keep
permissive CORS. INBUXA_HTTP_BASIC_AUTH=all puts it back everywhere;
dav is the default, and any other value logs a warning and keeps it.
Test builds accept Basic everywhere, since the integration suites sign
in with passwords, and legacy_protocols.py sets the variable.

Tested: unit tests for the paths, and tests/e2e/http_basic_auth.py
against the debug build, 26 checks, including both front ends' sign-in
path and a refused unregistered redirect for an ordinary account.

tests/e2e/legacy_protocols.py

@@ -90,7 +90,9 @@ def start(env_file=None):
             "-p", f"127.0.0.1:{PORTS['submissions']}:465",
             "-p", f"127.0.0.1:{PORTS['imap']}:993",
             "-p", f"127.0.0.1:{PORTS['pop3']}:995",
-            "-p", f"127.0.0.1:{PORTS['smtp']}:25"]
+            "-p", f"127.0.0.1:{PORTS['smtp']}:25",
+            # This script signs in with passwords over JMAP (contract C-23).
+            "-e", "INBUXA_HTTP_BASIC_AUTH=all"]
     if env_file:
         args += ["--env-file", env_file]
     args += ["stalwartlabs/stalwart:v0.16.22", "--config", "/etc/inbuxa/config.json"]