This makes phpcs stricter (not warnings allowed) and, also,
enabled phpdoc tests.

That way we have all the moodle-plugin-ci tests passing for
the repository and ready to check any future improvement to it.

Fixes moodlehq#13

Also, renew the php/branches combinations and remove some
deprecated stuff.

Just a bulk change, so we have all them moved.

Have kept the UselessOverridingMethod ignored, because
I only wanted to introduce here the minimum changes needed.
Basically:
 - superfluous MOODLE_INTERNAL checks.
 - deprecated print_error() uses.
 - Logical operators (or => ||).
 - small namespace issues in tests.

Also, updated to show the link to the matrix chat room

arrays syntax, test requirements, lang string ordering... nothing
critical, so far.

Also, add 404-STABLE to tests, part of MDLSITE-7646