Update ThemeController for backup/restore of individual files

This includes functions to check for modifications to the files
(images and scripts) that would normally be copied in by the theme.
The idea is that you ask the user which of the files they would like
to overwrite and which they would like to keep.  For the files they
want to keep, copy them to temporary locations before loading the
theme, then copy them back from the temporary locations.
Update theme views to prompt user for overwriting static files

Loading a theme would normally copy images and scripts to the /media/
dirs.  In many cases users have customized those files (e.g. using the
admin filebrowser) and don't want their changes to be overwritten.  The
typical case is a chapter's logo image.

The solution presented here is to create a new view, confirm_overwrite().
This view is called whenever a difference is detected between the theme
provided files and the files currently in /media.  In this case, the user
has to choose whether to keep or overwrite each file (the default is
keep).  With this form submitted, confirm_overwrite hands control back
to the previous view function (e.g. select() or recompile()) which tells
the ThemeController which files to keep.
Creates missing media dirs
Merge branch 'main' into themes-prevent-overwrite
Make ThemeController.backup_files() keep everything by default

This allows the ThemeController.recompile_theme(), clear_theme(), etc.
functions to be called with no keep_files argument, without overwriting
local changes to theme files.  The default behavior is now to detect and
preserve all local changes.
Add purge_all() function to clear Varnish cache
Clear Varnish cache after theme changes
Merge branch 'themes-prevent-overwrite' of github.com:learning-unlimited/ESP-Website into themes-prevent-overwrite
Merge branch 'main' into themes-prevent-overwrite
Merge pull request #977 from learning-unlimited/themes-prevent-overwrite

Prevent overwriting static files when loading themes