Improve the customizability of authentication screens

[mikejohnstn]

• Terms of service (TOS) text is not localized
• Set a custom TOS URL (maybe using localization?)
• Option to make the displaying of the TOS link optional
• Option to add the Forget password link
• Option to set the color of the Navigation controller (when authenticationOptional is active)
• Option to change the sign-in button colors
• Option to set the font type & size for the texts (especially for the UITextFields)
• The logo should not be hidden on an iPad, there is enough space to show all content

From #156 (comment)

[mikejohnstn]

Related: #153

[jleandroperez]

• Restrict the ability for users to sign up and only allow login.

Ref Issue #237

[danielr]

Just a thought: Rather than requiring to subclass SPAuthenticationViewController, you could introduce a protocol that includes authenticator and signingIn as required properties and to which any custom authentication view controller must conform to. So, basically the authentication view controller type would be something like UIViewController<SPAuthenticationUISupport>. A quick look at the code didn't reveal any further dependencies to SPAuthenticationViewController other than the two properties mentioned above.

This would allow to "start fresh", and thus also eliminate the risk of having weird and hard-to-debug side-effects caused by some superclass method (after all, SPAuthenticationViewController is quite a heavy view controller and does not seem to be specifically designed for safe subclassing).

[jleandroperez]

I agree, we should decouple the backend interaction from the SPAuthenticationViewController class, and offer a mechanism to implement authentication by means of a protocol. That would allow 3rd party apps to implement auth in (any imaginable) way.

Perhaps that could even allow you to tap into the new TouchID API, and feed the fingerprint-token as the user password.

Thanks for your suggestion @danielr!