Squashed 'docs/source/_themes/' content from commit 21e875d

git-subtree-dir: docs/source/_themes
git-subtree-split: 21e875d3a53ce897089ad690d897252f6063349d
Merge commit '94c258077e4b0870baa1ffd3350cedd3d1dd7159' as 'docs/source/_themes'
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 or creates a new talk
when the currently selected talk has unsaved changes, a window comes up
asking the user if they wish to save their changes, with the options
"Yes", "No", and "Cancel". "Yes" saves the changes and selects the new
talk; "No" selects the new talk without saving the changes; and
"Cancel" keeps the current talk selected without discarding or saving
any changes made.

Fix #501
Close #605
Customize button text and add prompt when user exits editor