Provide input for collecting repository URL in App Mining registration form

[markmhendrickson]

A new input for "Public repository" should appear when the user selects "Yes" for "Does your app have Blockstack Auth implemented?" in the App Mining registration form upon selecting "Register for App Mining" on https://app.co/mining.

This input should be displayed below the above prompt and above the "Submit" button. Its placeholder text should be "https://github.com/developer/app". The field should be optional and the value entered should be stored alongside the rest of the registration data in our database.

Requested by @xanbots

[aulneau]

@hstove I've added this to the frontend as an optional field. the key I chose is repo

[aulneau]

@markmhx I've opted to keep the input above the radio buttons as it felt out of place. I also think a placeholder of Public Repository (GitHub, etc)* makes more sense and is consistent with how we are using our other labels as placeholders. If you have a larger rational for either choice please let me know!

[markmhendrickson]

@aulneau Those are good improvements 👍🏻

As a result, though, we should probably just show the public repository input regardless of whether the user toggles "Yes" for the Blockstack Auth prompt, since it would be strange to reveal an input above that prompt.

[aulneau]

Yep! That's how it currently functions