Implement the new Privacy API

[scara]

MDL-61307 and MDL-61397 landed into the main stream (3.3, 3.4 and 3.5) in the current weekly (20180315): it's time to implement what required by the new Moodle Privacy API.

We do not store any personal data so we must implement the metadata null provider, as already done e.g. in the core Global Search block.

[scara]

The GDoc has been integrated in the Moodle Docs Wiki: new ref in https://docs.moodle.org/dev/Privacy_API#Plugins_which_do_not_store_personal_data .

[scara]

Unfortunately I've tried many hacks to allow sharing the same plugin code on different branches AND PHP versions with no success: https://travis-ci.org/scara/moodle-local_twittercard/jobs/364187213.

The main issue is the fact that the Privacy API is different in 3.4 since it declares the return type in the null provider which is not supported in PHP 5.6, potentially used in 3.3: I'll introduce a separate branch to keep on supporting Moodle 3.3 (which is actually an effort).

[scara]

A providential update to the Moodle Privacy API docs should get rid of all my past attempts to create a safer 3.3 only plugin branch: https://docs.moodle.org/dev/Privacy_API#Difference_between_Moodle_3.3_and_more_recent_versions .

TNX @abgreeve! 👍