MDL-39152 webservices: Rename external_api::get_context() to external_api::get_context_from_params()

Since many apis in future will need to provide alternative method to fetch context, I have abstracted this bit into the core external_api class

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.

MDL-12886 refacoring: the external_param is in fact value because we use it also in return structures