fix #549: add option to remember last share selection

* add checkbox in share view to remember last share selection.
* this checkbox saves state differently for text shares and image shares.
* add a "reset auto-sharing settings" in preference view.
* if the select blog id doesn't exist anymore (or is hidden), auto-sharing settings will be reset, the user notified by a toast message.
Merge branch 'develop' into issue/549-add-option-to-remember-last-share-selection
change checkbox style to match the dark background
better wording
Merge pull request #570 from wordpress-mobile/issue/549-add-option-to-remember-last-share-selection

Fix #549: add option to remember last share selection