MDL-46093 quiz review should default to showing everything on one page

... as long as the quiz is not too big. At the moment, that limit is
set to 50 questions.
MDL-32450 delete Dragmath plugin and library because it is not compatible with GPL
MDL-32450 upgrade: do not remove DragMath settings if the files still exist
MDL-40457 Question Bank: Allow plugins to add columns to question bank view

Allows plugins to add columns to the question bank view by extending core_question\bank\column_base
Columns to display are set in $CFG->questionbankcolumns. Columns are namespaced and autoloaded to support this.
MDL-42071 caching Support for clustered memcached caching

For stores where there is a very high rate of sets compared to gets, it
is beneficial to retrieve from the local server, skipping the network
overhead, at the expense of having to set many severs when a key is set.

This patch adds a memcached option to enable clustering. When on, only
one "server" is allowed, which will be where fetches are from, while
sets/updates/deletes/purges will occur to the all the servers in the
"set server" list.

To run unit tests, define TEST_CACHESTORE_MEMCACHED_TESTSERVERS with
multiple (return delimited) servers.
MDL-42071 caching Support for clustered memcache caching

For stores where there is a very high rate of sets compared to gets, it
is beneficial to retrieve from the local server, skipping the network
overhead, at the expense of having to set many severs when a key is set.

This patch adds a memcache option to enable clustering. When on, only
one "server" is allowed, which will be where fetches are from, while
sets/updates/deletes/purges will occur to the all the servers in the
"set server" list.

To run unit tests, define TEST_CACHESTORE_MEMCACHE_TESTSERVERS with
multiple (return delimited) servers.
MDL-32450 editor_tinymce: removed DragMath from the 'customtoolbar' setting
MDL-32450 editor_tinymce: documented removal of DragMath
MDL-46099 session: fix use of references for session globals

This reverses the references used for global $USER and $SESSION,
the reason is that PHP does not allow references to references.
$USER is a reference to $GLOBALS['USER'] which means we cannot
put any references to it. Solution is to store the current user and session
objects in $GLOBALS['USER'] and $GLOBALS['SESSIOn'] are reference
them in $_SESSION.

This patch makes the session code behave the same way in CLI,
phpunit and normal web requests - this allows use to finally
unit test most aspects of the session code in Moodle.
Merge branch 'MDL-42071_MK3' of https://github.com/merrill-oakland/moodle
Merge branch 'MDL-40457-question_bank_plugins_columns' of git://github.com/MorrisR2/moodle
Merge branch 'MDL-32450_master' of https://github.com/markn86/moodle
Merge branch 'wip_MDL-46099_m28_userglobal' of https://github.com/skodak/moodle
Merge branch 'MDL-46093' of git://github.com/timhunt/moodle