Improve Talk Overlay

* Fix "close TalkOverlay when TalkSectionOverlay is closed" by call .show()
after TalkSectionOverlay or TalkSectionAddOverlay is closed.

* Added header to TalkSectionOverlay and TalkSectionAddOverlay to show
back instead of close button.

* Show Title of Talk as Title (of Overlay) in TalkSectionOverlay instead of
a seperate line.

* Use named button in TalkSectionAddOverlay (like Editor) instead of image.

* Cursor for links in talk overlay should be a pointer.

* Fixed "Uncaught TypeError: Cannot read property 'hide' of undefined" after
adding a new discussion by remove the try to hide talkOverlay. Main problem addressed
in change I2a31a8b704adc2401e1b2c030cfe54a0eb7c771e

Bug: 64105
Change-Id: If76138eaa015607241bad2ec773d48cf2daf650a
Merge "Improve Talk Overlay"