MDL-47603 quiz edit: qbank popup preview icon needs new window

Also, ensure that clicking the + icon adds the question on the right
page.

Bit of this fix are a bit hacky, but we have to deal with the fact there
is not a good general solution to MDL-47604.
MDL-46819 core_grades: Negative weights are changed to 0

Negative weights in the natural aggregation are not currently permitted.
Any figure below zero is changed to a zero.
MDL-46546 core_grades: Notice display for bad import.

If the initial import of csv data is bad, a notification
will be displayed to the user.
MDL-47140 editor_atto: Improved error handling
Merge branch 'MDL-47140-master' of git://github.com/gurgus/moodle
Merge branch 'wip-MDL-46546-master' of git://github.com/abgreeve/moodle
Merge branch 'wip-MDL-46819-master' of git://github.com/abgreeve/moodle
Merge branch 'MDL-47603' of git://github.com/timhunt/moodle