[Auth] Remove the .showLinkMailView segue and navigate programmatically

[mindgraffiti]

This PR removes the segue that sends a user to the "Magic Link sent" screen and updates the programmatic navigation for opening the app after a Magic Link has been clicked.

See also: wordpress-mobile/WordPressAuthenticator-iOS#217

To test:

1. Check out this branch
2. rake dependencies
3. Build and run
4. Navigate through the Login flow (listed below)

Login Flow

1. Select the Log in with Jetpack button
2. Enter a valid email address that can use Magic Links, select the Next button
3. Select the Send Link button
4. Verify you are navigated to the "Magic link sent" screen
5. Verify you received an emailed magic link
6. Open the magic link
7. Verify the magic link navigates you to the "Continue" screen

Do not merge until:

1. The Authenticator PR has been merged: Remove segue .showLinkMailView wordpress-mobile/WordPressAuthenticator-iOS#217 ✔️
2. A new Authenticator release has been created ✔️
3. The release is published on Cocoapods trunk ✔️
4. This PR points to the new release ✔️

Update release notes:

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-woocommerce]

You can trigger an installable build for these changes by visiting CircleCI here.

[jaclync]

Login with magic link worked nicely 👍

[pmusolino]

Thanks @mindgraffiti!

[mindgraffiti]

Thank you for the reviews @jaclync and @pmusolino !