#508 Change Add New Talk button to a command button

- Move 'Add New Talk' button from TalkDetailsWidget to CommandButtons

- Connect new 'Add New Talk' button to confirm_add

This change is based on the upcoming functionality for the Add New Talk button, which will involve adding an empty talk (no metadata) to the database.

Fixes issue #508
Related to issues #427, #494, #500

Related proposal:
https://docs.google.com/document/d/1VdPLpdIgteMfr1l7TPIZQ1M5XUKcy7ffVWu41t5zvjk/edit#