MDL-44070 Conditional availability enhancements (2): subsystem, API

This commit defines the new /availability root folder, with
/availability/classes, /availability/tests, and
/availability/condition where the condition plugins will live.
Condition plugin prefix is availability_, e.g. availability_date.

Rationale for this organisation:

1. I was originally going to put this in /lib/availability but
   it has been pointed out that putting even more junk in lib
   is probably bad.
2. 'availability' and 'condition' are the two names used in code
   to refer to this system ($CFG->enableavailability).
3. The prefix has to be short enough to allow database tables
   (although in practice I assume that condition plugins will not
   normally contain database tables).

The new API includes a Boolean tree structure that controls the
availability of an item.

AMOS BEGIN
 CPY [availabilityconditions,core_condition],[restrictaccess,core_availability]
 CPY [enableavailability,core_condition],[enableavailability,core_availability]
 CPY [configenableavailability,core_condition],[enableavailability_desc,core_availability]
AMOS END
MDL-44070 Conditional availability enhancements (3): conditions

Includes all the conditions that were in previous Moodle versions:

* Date
* Grade
* Completion (of another activity)
* User profile field

Also includes conditions that are used to reimplement
groupmembersonly:

* Grouping
* Group

For each condition, the component plus unit tests are included.

PLEASE NOTE: The code to actually check each condition is reused
from previous Moodle versions and has not been modified except to
pass codechecker. This is intentional, to reduce the risk of the
change and maximise the chance that behaviour is preserved. Some
of this code might not be very good and might need updating but
that can happen separately.

AMOS BEGIN
 CPY [contains,core_condition],[op_contains,availability_profile]
 CPY [doesnotcontain,core_condition],[op_doesnotcontain,availability_profile]
 CPY [endswith,core_condition],[op_endswith,availability_profile]
 CPY [isempty,core_condition],[op_isempty,availability_profile]
 CPY [isequalto,core_condition],[op_isequalto,availability_profile]
 CPY [isnotempty,core_condition],[op_isnotempty,availability_profile]
 CPY [startswith,core_condition],[op_startswith,availability_profile]
 CPY [completion_fail,core_condition],[option_fail,availability_completion]
 CPY [completion_pass,core_condition],[option_pass,availability_completion]
 CPY [completion_complete,core_condition],[option_complete,availability_completion]
 CPY [completion_incomplete,core_condition],[option_incomplete,availability_completion]
AMOS END
MDL-44070 Conditional availability enhancements (4): backup

Implements backup and restore for the new conditional availability
data. This includes:

* Backup and restore of new field.
* Restore updates IDs in conditions, as required.
* Restore converts availability data from legacy (Moodle 2.6) backups.
* Unit tests for all this.
MDL-44070 Conditional availability enhancements (5): deprecate old lib

The old conditionlib.php, which is replaced by the new availability
API, has been deprecated.

* Code which uses the key functions (is_available,
  get_full_information) should still work. These now show
  deprecated warnings and then pass through to equivalents in the
  new API.
* I have created new unit tests for these functions.
* The old language file has been removed (reused strings already
  moved with AMOS).
* Most other functions throw exceptions because it was impossible
  (due to fundamental API differences) or difficult to reimplement.
* I don't really expect that non-core code (outside unit tests) will
  have used any of the other functions.
MDL-44070 Conditional availability enhancements (6): core changes

Changes core code to use new API instead of the old one when
checking user access to activities and sections.

Includes changes to other libraries that are necessary after
adding the availability system and removing old conditional tables
etc.
MDL-44070 Conditional availability enhancements (7): forms

Changes to the module and section editing forms to use the new
fields and JavaScript.
MDL-44070 Conditional availability enhancements (8): display

Includes theme and renderer changes required for the new feature.
There are two parts: style required for the display as part of the
course page (when showing the list of conditions for an activity),
and style required for the settings form.

This change supports both the bootstrapbase theme (which was the
main theme used during development) and the old base theme.
MDL-44070 Conditional availability enhancements (9): wider changes

Changes to arbitrary areas of code that are necessary after the new
API (mostly to replace deprecated function calls).
MDL-44070 Conditional availability enhancements (10): autogenerated

Includes all the autogenerated files (to make it easier/less worrying
when reviewing and rebasing).