[patch] Make the site block check usable from other plugins and core modules

Please add an API to check if an address is blocked.

The patch adds method SiteManager.isAddressBlocked(), that can be overloaded by filter plugins to provide the actual implementation. It also adds the corresponding code to ContentFilterPlugin and moves a few lines from ContentFilterPlugin to ContentFilterStorage to avoid the code duplication.

[isAddressBlocked.txt](https://github.com/HelloZeroNet/ZeroNet/files/2827088/isAddressBlocked.txt)