Improvements to add discussion on talk page

* Use right top button to add a new discussion, instead of button in
content-header (same as in editor "continue").
* reload page after the new discussion is saved
* Ask for confirmation when leave the Add discussion overlay and
content is in the textfields.
* Add QUnit tests for TalkSectionAddOverlay
* Remove the try to hide parent Overlay when a new comment was saved to
a discussion (prevent TypeError)

Bug: 69763
Change-Id: I2a31a8b704adc2401e1b2c030cfe54a0eb7c771e