Replace calls to \coursecat::get_all() or cache the results.

- Define sitepolicy handler manager class, base class and the core handler
- Allow to set a plugin as sitepolicyhandler that implements the sitepolicy API
- Modify web services to return information from the 3rd party handler instead of core if needed

David Mudrák authored Jan 24, 2018 and Marina Glancy committed Mar 15, 2018

The patch introduces a new site setting 'sitepolicyhandler' that can be
either empty or contain a plugin component name. If it is empty, the
site policy feature keeps working as before and the core keeps the
control over it.

If a plugin is specified, it is expected to implement the callback
'site_policy_handler' in its lib.php file. The callback should return a
URL to a script where the user can accept the site policies. The plugin
itself is responsible for setting the 'policyagreed' flag in the users
table. The callback may return an empty value, in which case the user
can continue using the site without being redirected.

The patch adds support for a new pre-config flag NO_SITEPOLICY_CHECK.
This constant should be defined and set to true if we should not check
the user's policyagreed status during the require_login().

This is for pages where the user actually accepts the site policies and
helps to avoid the redirect loop.

Instead of creating an array containing all record information and then
writing, we know write periodically during the recordset iteration, thus
alleviating any memory concerns associated with the array approach.

* Notify administrators once incoming IPN request is verified by PayPal.
* Fix the HTTP status as expected by the IPN protocol.

Without this patch, users could still log in as a suspended account via
OAuth2 SSO feature.

Merge branch 'm35_MDL-61626_PgSQL_Table_Name_Could_Be_Schema_Qualified' of https://github.com/scara/moodle