Only run dapp detection if metamask is not installed

[bbondy]

No description provided.

[bbondy]

0.19.x: 911febc
0.20.x: edabfa0
0.21.x: 3113e67
master: 843bcc6

[kjozwiak]

STR/Test Cases:

Case 1:

• ensure MetaMask is disabled via about:preferences#extensions
• visit https://ethwaterloo.herokuapp.com/
• 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)
• click on the "MetaMask" icon that appears next to the "Brave" shield icon
• accept the terms and quickly create a new account
• click on "I've copied it somewhere safe" under "Vault Created"
• select the "Ropsten Test Net" from the MetaMask dropdown menu
• under https://ethwaterloo.herokuapp.com/, refresh the page and select "Go" and ensure the app loads a blockchain map

Case 2:

• ensure MetaMask is disabled via about:preferences#extensions
• visit https://leeroy.io/
• ensure that you receive a "This page contains a Dapp, would you like to enable the MetaMask extension?" notification
• ensure that you can't sign up
• click on "Install MetaMask"
• ensure that MetaMask is installed (verify via about:preferences#extensions)
• unlock MetaMask and ensure that you can now type a username/click on "Sign Up"

Case 3:

• ensure MetaMask is disabled via about:preferences#extensions
• visit https://ethwaterloo.herokuapp.com/
• ensure that you receive a "This page contains a Dapp, would you like to enable the MetaMask extension?" notification
• click on "No Thanks" and ensure "Remember this decision" is selected
• refresh the website and ensure that the notification isn't being displayed anymore
• visit https://tokenfactory.surge.sh and ensure that the notification isn't appearing

Case 4:

• ensure MetaMask is enabled via about:preferences#extensions
• visit https://ethwaterloo.herokuapp.com/ and ensure that the "DApp" notification isn't being displayed
• visit https://tokenfactory.surge.sh and ensure that the "DApp" notification isn't being displayed

Case 5:

• ensure MetaMask is disabled via about:preferences#extensions
• visit https://brianbondy.com/, https://www.facebook.com, https://www.twitter.com
• ensure that the "DApp" notification isn't being displayed