Fix #571: Footer broken on RTD site
Remove stuff that's not needed for the theme
Update copyright

The copyright in the build configuration file shows in the default
footer.
Hide default footer in docs by hiding default copyright

The default footer only consists of the copyright statement, so we
can get rid of the footer by not showing the copyright.

Instead we show a custom footer thanks to @umcsdon.
Fix #501 Warn user about losing new talk info

When the user switches from one talk to another, exits the talk editor,
or creates a new talk when the currently selected talk has unsaved
changes, a window comes up warning the user that there are unsaved
changes. The options are "Save Changes", "Discard Changes", and
"Continue Editing".

Fix #501
Close #605