Remove rendered Blades on theme install/uninstall

[alwaysblank]

This PR would address some of the concerns in #1987. It doesn't add a command-line function for manipulating Blades, but it does make Sage a slightly kinder WordPress citizen by cleaning up Blades when installed/uninstalled, and provides some helper methods for doing that at other times. It endeavors to be relatively careful, and only delete the files it needs to delete, leaving other files alone.

The reason it doesn't examine existing Blade templates to find their rendered versions and delete those is because part of the ostensible purpose is to clean up unnecessary Blades when changing themes: If the original Blade templates have been changed or removed during development, the "orphaned" rendered files would not be found.

I considered adding the a "build all Blades on install" feature, but felt that was a little too presumptuous and could theoretically lead to significantly increased load when activating a theme if there are a lot of Blades to render. I'd be happy to modify the PR to add that feature, though.

[retlehs]

i know this is old, but can we re-do this for sage 10?

[Log1x]

No longer needed as of #2174