This fixes the ability to pass custom theme directories to
the management command which compiles site themes, a la:
./manage.py lms compile_sass --theme-dirs /my/custom/themes/dir
The exception, which was due to a incompatible use of @lru_cache, was:
File "openedx/core/djangoapps/theming/management/commands/compile_sass.py",
line 93, in parse_arguments:
available_themes.update({t.theme_dir_name: t for t in get_themes([theme_dir])})
TypeError: unhashable type: 'list'
This has been broken since the @lru_cache decorator was added, but it
wasn't noticed because:
* We weren't compiling any comprehensive themes in CI.
* Tutor supports compehensive theming, but not *site theming*, so
it doesn't use this management command at all
(site themeing == comp theming * site configuration).
* Although edx.org executes this management command, it does not provide
use the `--theme-dirs` argument, so the bug was not hit.
5fe131c858