luci-app-adblock: User interface for the 'adblock' package

Simple user interface for the 'adblock' package from packages feed.
Adds 'Adblock' item to the Services menu in LuCI.
Allows enabling/disabling the service and selecting blocklists.

Configuration of the blocklist URLs and selection of Shalla categories
needs to be done manually to the config file, if needed.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>