add notification that installs MetaMask when visiting DApp websites

[kjozwiak]

Description

When a user visits a DApp website, Brave should prompt the user if they would like to enabled the MetaMask extension if it's currently disabled via a notification.

Steps to Reproduce

• ensure MetaMask is disabled via about:preferences#extensions
• visit https://tokenfactory.surge.sh
• ensure that you receive a "This page contains a Dapp, would you like to enable the MetaMask extension?" notification
• click on "Install MetaMask"
• ensure that MetaMask is installed (verify via about:preferences#extensions)
• once installed, refresh visit https://tokenfactory.surge.sh
• ensure that you can see the "Block Numbers" being updated

Brave Version

about:brave info:

Brave: 0.19.49
rev: 6795934
Muon: 4.4.28
libchromiumcontent: 61.0.3163.100

[darkdh]

closed by #11534