Remove segue .showLoginEmail

[mindgraffiti]

Fixes #205
Ref. #182

This PR removes all references to the segue .showEmailLogin and programmatically navigates the user to the LoginEmailViewController. There are 3 areas where the segues have been removed:

1. WPiOS sign up
2. WPiOS sign in
3. WCiOS sign in

To Test - WPiOS

1. Check out this branch
2. bundle exec pod install and ensure there are no errors
3. Build and run (there should be no errors)
4. Visit the WPiOS PR to run the changes: Remove the .showLoginEmail segue and replace with programmatic nav WordPress-iOS#13669

To Test - WCiOS

1. Check out this branch
2. bundle exec pod install
3. Build and run (there should be no errors)
4. Visit the WCiOS PR to run the changes: [Auth] Remove the .showLoginEmail segue and navigate programmatically woocommerce/woocommerce-ios#2007

[ScoutHarris]

Hey @mindgraffiti .

The signup to login flow is a bit different than develop. (Sign up for WordPress.com > Sign up with Email > Enter an email address that has an existing WP.com account)

1. develop auto-populates the previously entered email address (thus auto-enabling the Next button).
2. develop does not show the site address link. And I don't see that this has changed in the Unified design.

develop	branch

[mindgraffiti]

Thanks @ScoutHarris! I forgot to remove the segue from the signup storyboard that navigated to the EmailLoginViewController 🤦‍♀. Review this commit: 01725ec and try to build and run. It should be good to go!

[ScoutHarris]

Looks good!

[diegoreymendez]

Sorry I didn't get to review this. From my end, feel free to move ahead since @ScoutHarris already reviewed.

[mindgraffiti]

Thanks @ScoutHarris !