This functionality no longer makes sense inside Moodle now that we have
external tools like stylelint to help us with this. It was not able to
be used by any core themes.

The code was always marked as not for external use.

Invalid CSS with extra } symbols, which previously (by fluke)
worked, was broken by recent chunking improvements. This change
makes the chunking code robust against this (stupid) situation.

This commit makes the following changes:
* Tidies up coding style.
* Tidies up phpdocs.
* Improve CSS optimiser unit tests to cover browser hacks.
* Improve backgound style consolidation.
* Improve border style consolidation.
* Fixed optimiser to handle browser hacks like *zoom.
* Improved @ rule unit tests.
* Fixed @rule breakages caused by OS specific rules.
* Added more unit tests to cover a bit more CSS3.

I am pretty sure this patch will now enable to optimsier to
work on bootstrap based themes should developers not wish
to compress thier compiled less CSS.

MDL-33468 css_optimiser: Fixed aggressive optimisation so that it only combines siblings and doesn't reorder

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

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

* Fixed reversed border styles.
* Added support for out of order background styles.
* Added more CSS tests again.
* Added better validation of CSS widths

MDL-29941 csslib: Improved unit tests and fixed bugs with background and border partial style optimisation