Add an admin endpoint to clear the entire cache

[KyleAMathews]

It'd be convenient if e.g. disk space is running out or if there's some bug in shape creation that people need to work around. It's a bit of a last resort type tool but very handy when you need it.

[msfstef]

This would be sort of like our DELETE shape endpoint, but it would delete all active shapes, right?

[KyleAMathews]

Right yeah, basically a clear the entire cache when a server goes wonky.