Implement ad blocker

Closes #1 (closed)

How to test:

flatpak-builder tmp --force-clean --ccache org.kde.mobile.angelfish.json --install-deps-from=flathub
flatpak-builder --run tmp org.kde.mobile.angelfish.json angelfish

or

• Install rust stable (distro packages, rustup)
• Compile https://github.com/AndrewGaspar/corrosion
• Compile angelfish as usual.

To do:

• Remove some debug output
• decide which lists should be default
• Allow to disable the ad blocking at runtime
• Consider making use of https://github.com/brave/adblock-rust/pull/127 later
• Update flatpak recipe