[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)

Start with the attached isAddressBlocked.txt patch and trace SiteManager.isAddressBlocked() into ContentFilterPlugin and ContentFilterStorage. Review how the existing content-filter check is shared, then verify that plugins and core modules can call the new API without duplicated logic.

Escrito por el modelo de indexación a partir del texto del issue.