PARAM_NUMBER is a float, our id's are int's

* Fix core_cohort_get_cohorts to specify correct return type on cohort id
* Fix core_enrol_get_enrolled_users_with_capability to specify correct
  correct return type on user id
* Fix core_cohort_update_cohorts to specify the right parameter type
  (int) on cohort id
* Add tests to verify the bug on core_cohort_update_cohorts param type.

As per our policy webservices should never have a required contextid, thuse providing a support for passing contextlevel and instanceid as parameters to the api

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

As per our policy asking for and returning contextids is discouraged in webservices. So provide alternative parameters for the same

MDL-32581 support additional format field with all text field in extrnal lib function + new possible ws params to decide how format is returned (which are stored in a singleton)

Conflicts:

	group/externallib.php

MDL-29347 core_enrol_get_enrolled_users web service should limit the number of participants - Jerome/Dongsheng

1. moodle_enrol_get_users_courses should return number of participants,
2. moodle_user_get_users_by_courseid should limit the fields to be returned

MDL-27564 most parameter should have been optional - return more common information so no extra web service required - note that it could be good to return role and group for each user in each course (it would make sens and save some web service call)

webservice MDL-23943 fix moodle_enrol_get_enrolled_users (return function, return value, sql result not unique, param name never used) + add unit test

webservice MDL-23440 more explicit error message when validation context fail in some external functions.

MDL-21782 reworked enrolment framework, the core infrastructure is in place, the basic plugins are all implemented; see the tracker issue for list of unfinished bits, expect more changes and improvements during the next week

AMOS START
    MOV [sendcoursewelcomemessage,core_admin],[sendcoursewelcomemessage,enrol_self]
    MOV [configsendcoursewelcomemessage,core_admin],[sendcoursewelcomemessage_desc,enrol_self]
    MOV [enrolstartdate,core],[enrolstartdate,enrol_self]
    MOV [enrolenddate,core],[enrolenddate,enrol_self]
    CPY [welcometocourse,core],[welcometocourse,enrol_self]
    CPY [welcometocoursetext,core],[welcometocoursetext,enrol_self]
    MOV [notenrollable,core],[notenrollable,core_enrol]
    MOV [enrolenddaterror,core],[enrolenddaterror,enrol_self]
    MOV [enrolmentkeyhint,core],[passwordinvalidhint,enrol_self]
    MOV [coursemanager,core_admin],[coursecontact,core_admin]
    MOV [configcoursemanager,core_admin],[coursecontact_desc,core_admin]
    MOV [enrolledincourserole,core],[enrolledin...