Purge cache when Custom CSS / header/footer code are saved.

Relies on these two new hooks:

https://github.com/OM4/om4-custom-css/commit/8bc33e10cb6d1ddc31327d9e0dd2a88c4d77d5bf
https://github.com/OM4/om4-header-footer-code/commit/2c6a161a3bbe1ce1d66f3446f68508560879b2e6
Revisions documentation.