Add a confirmation dialog when clicking on the big tick

See #519
Before, when clicking on the big tick to mark all articles as read, it triggers directly the action.
Now, it asks for a confirmation to block unwanted user clicks.
Add confirmation on button as well
Add a configuration parameter

Add a parameter to choose wether or not the user want to display a confirmation dialog when clicking on "mark all as read" actions.
Merge pull request #531 from aledeg/issue519

Add a confirmation dialog when clicking on the big tick