Add support for Metamask login

[joaosantos15]

Currently on the dashboard, login is done by having the user sign a Metamask message, instead of providing username and password.

To support this, Mythxjs should:

• Make a /auth/challenge to the mythx API
• Construct the Metamask challenge and have Metamask prompt the user
• Get the signature from Metamask
• Send that signature to the auth/login endpoint on the mythx api

[joaosantos15]

cc @Gtonizuka

[Gtonizuka]

There is a PR waiting to be merged in that solve this issue