REST API: Add A/B experiment for logging in with application password

[itsmeichigo]

• Update prologue screen to display only entry point to site address login New configuration for site address login only on the prologue screen wordpress-mobile/WordPressAuthenticator-iOS#725
• Redirect to WPCom login if detect WPCom site, otherwise show site credential login REST API: Update login flow for AB testing the REST API project #8684
• Add new error screen for application password disabled REST API: Update login flow for AB testing the REST API project #8684
• Design experiment and post a proposal P2. pbxNRc-2i4-p2
• Track which variant the user is assigned to (pe5sF9-13h-p2) [REST API] Track assigned A/B test variant #8753
• Replace feature flag applicationPasswordAuthenticationForSiteCredentialLogin with the check for the A/B experiment. [REST API] Introduce A/B test and remove feature flag #8744

[selanthiraiyan]

A/B test proposal, Abacus and PR work done.