Admins can clear the image cache

[mcpierce]

As a library owner, I would like to clear all entries from the image cache without having to manually delete the directory or access the backend server.

[bareheiny]

Could this be split into two processes:

1. Delete everything
2. Delete everything except covers

I like the responsiveness from having the covers cached, but don't want my covers directory filled up with the other pages in a comic - especially if I open a 500 page trade.

[mcpierce]

@bareheiny Potentially, but we'd need to somehow indicate in the cache that a file is a cover vs a story page to do that. Maybe as a follow on enhancement to this ticket.