Access SAPI 5 via the pythoncom/win32com modules

Removes the Python code that interacted with cscript and the JSript glue
script and instead attempts to do this all in Python with the pythoncom
and win32com modules instead.
Drop disused SAPI 5 JScript gateway code

Also adjusts the migration code and two helper scripts.
Workaround for gen_py path issue on some installs

Some Anki installations will not have a working __gen_path__ when
starting up, so we can fix this by repointing it at the user's temporary
directory.
New configuration option for cache limit

Adds a new cache_days configuration options that a user may change. This
will be used to determine how old a file in the cache may be before it
is dropped.
Implement cache control hook

This is based on unloadProfile and will do scans of the cache directory
based on the user's cache_days setting.
Documentation update for cache control

Updates text about cache settings and updates screenshots of the
Advanced configuration tab.
Merge branches add/cache-control, bug/sapi-victor

Brings in an improved JScript-less implementation for SAPI and adds
automatic cache control.