MDL-43325 JavaScript: Set a default visibility for most Moodle dialogues

We used to ignore the 'visible' field in the initializer, but now we hide
dialogues if visible is false. This was an unintentional change from the
stable API and means that confirmation boxes, alerts, and exceptions are no
longer shown by default.
MDL-34182: Add functions to warn of output in AJAX scripts

We need to be able to capture output when including contributed code as it
may add additional content which makes makes the json we output invalid.

We only warn of these errors if the site is in debugdeveloper, otherwise
we silently drop them.
MDL-34182 AJAX: Change to use shared method when checking the siteadminbranch for invalid output
MDL-34182 Repositories: Check for and warn on invalid output in the file picker
MDL-43318 Assign: Fix bug when editing previous attempt feedback
Merge branch 'MDL-34182-26' of git://github.com/andrewnicols/moodle into MOODLE_26_STABLE
Merge branch 'MDL-43325-26' of git://github.com/andrewnicols/moodle into MOODLE_26_STABLE

Conflicts:
	lib/yui/build/moodle-core-notification-dialogue/moodle-core-notification-dialogue-min.js
MDL-34182 js: fixed typo in ajaxException use
Merge branch 'MDL-43318-26' of git://github.com/damyon/moodle into MOODLE_26_STABLE