This is used by messaging system for sending/receiving message
to/from noreply or support user. message_send api will now use
core_user class to get noreply or support user and then
send/receive message depending on user state.

Includes:
* no more hacky reloads, everything is written only once and kept until cache reset
* lang menu list is now cached in MUC
* both string and lang menu caches are compatible with local caches on cluster nodes
* config-dist.php cleanup

MDL-40627 add 2G extramemorylimit option to admin UI and use the same units in config-dist.php to prevent confusion

Credit goes to Julien Boulen, thanks for spotting this.

Make "Advanced" and "Lockable" flags for any admin setting.

- 'List of courses' is split into 'List of courses' (available) and 'Enrolled courses', CFG->disablemycourses is deprecated;
- CFG->frontpageloggedin by default shows list of available courses;
- There is separate item to display course search box
- CFG->maxcoursesincombo is deprecated
- CFG->maxcategorydepth changed default value to 2 since we have AJAX loading now
- FRONTPAGECOURSELIMIT is transformed to CFG->frontpagecourselimit

c

Moving from phpunit test environment
to a specific behat test environment

Also more code comments and general cleanup

Includes X-Sendfile support, ETags for stored_files and general cleanup.

Includes:
* constants refactoring
* reworked db table init
* support for $CFG->debug = -1
* functional DB tests
* fixed $DB->get_indexes() to not throw exceptions when table does not exist
* fix handling of user passwords in test db
* add debug info to exception messages
* removed unnecessary PHP debug errors from mathslib
* fixed @error suppression in get_string
* fixed PHPUnit error handler setup
* added timezone info to default install

Please note that this setting can not be stored in database because some scripts use ABORT_AFTER_CONFIG.

Thanks Eloy Lafuente, Tim Hunt and Sam Hemelryk for valuable feedback and ideas.