api: Add a monotonic integer "feature level" for non-webapp clients.

The purpose is to provide a way for (non-webapp) clients,
like the mobile and terminal apps, to tell whether the
server it's talking to is new enough to support a given
API feature -- in particular a way that

* is finer-grained than release numbers, so that for
features developed after e.g. 2.1.0 we can use them
immediately on servers deployed from master (like
chat.zulip.org and zulipchat.com) without waiting the
months until a 2.2 release;

* is reliable, unlike e.g. looking at the number of
commits since a release;

* doesn't lead to a growing bag of named feature flags
which the server has to go on sending forever.

Tweaked by tabbott to extend the documentation.

Closes #14618.

zerver/views/auth.py

@@ -45,7 +45,7 @@ from zproject.backends import password_auth_enabled, dev_auth_enabled, \
     ldap_auth_enabled, ZulipLDAPConfigurationError, ZulipLDAPAuthBackend, \
     AUTH_BACKEND_NAME_MAP, auth_enabled_helper, saml_auth_enabled, SAMLAuthBackend, \
     redirect_to_config_error, ZulipRemoteUserBackend, validate_otp_params
-from version import ZULIP_VERSION
+from version import ZULIP_VERSION, API_FEATURE_LEVEL
 
 import jwt
 import logging
@@ -946,6 +946,7 @@ def api_get_server_settings(request: HttpRequest) -> HttpResponse:
     result = dict(
         authentication_methods=get_auth_backends_data(request),
         zulip_version=ZULIP_VERSION,
+        zulip_feature_level=API_FEATURE_LEVEL,
         push_notifications_enabled=push_notifications_enabled(),
         is_incompatible=check_server_incompatibility(request),
     )