MDL-40412 course Expanding the height of module description fields.

Expanding the default height of mod descriptions to 10, which gives
you 3 real lines once TinyMCE Loads.
MDL-39812: bootstrapbase theme: Fix breadcrumbs layout for narrow screen
MDL-39812 clean theme: fixed breadcrumbs layout for narrow screens
MDL-40195: mod_assign: Display grades when quickgrading to 2 decimal places
MDL-40538 caching: Use isset for performance

isset is much faster than array_key_exists. For caches we need to
ensure as much performance as possible. To all null to continue to
be stored and provide correct results, non ttl also now saves in
$this->store[$key][0].  As isset($this->store[$key]) will return
true in all cases.

time() as found in cache::now() can be slow on some platforms and
we shouldn't bother checking it unless needed. It also stops non-ttl
caches paying the ttl price.

Unit tests were adjusted to ensure null is still handled correctly
when we are only using isset.
MDL-38857 tool_behat: Adding info about the tool purpose
MDL-38794 passed overflowdiv parameter
MDL-38747 blocks: course format options 'numsections' is required to display the block
MDL-40536 course: unit tests for course_external::import_course

Basic import operations and a specific test case which identifies
a bug.
MDL-40536 course/externallib fix incorrect variable

Fixes test_import_course_invalid_deletecontent_option() from the
previous commit.
MDL-40569 fix empty CSS files after cache purging
MDL-40558 skip admins and guest when deleting incomplete users
MDL-40633 filter_urltolink breaking some image links.

This was a regression caused by MDL-22390. HTML like
<img src="http://www.example.com/logo.gif" />
was being broken.
MDL-40423 fix creation of new install.xml in subplugins
MDL-40571 work around block editing hacks on admin page
MDL-40611 fix context cache size expectation
MDL-40621 fixed small bug in function editor_tinymce_plugin::fix_row()
MDL-40652 Calendar: Changed pagelayout from 'standard' to 'base' in calendar/export.php.
MDL-39062 cache: persist keys now set properly
MDL-39062 cache: version bump to update definitions