Add FreeOTP app logo in 2Fa exercises

Add the logo of the application in the Introduction page (first page) of the 2FA exercise)

• File: http://www.feirox.com/rivu/2016/06/FreeOTP-Authenticator-4.png
• Reason: because the app has several names depending on the origin on the download (FreeOTP+, Free OTP Authenticator), plus other apps have very similar names on the app stores.
• Result: the logo on the right side of the screen at the level of the three app stores URL
• In all available versions (EN, FR)