Highlight the ability to use an existing account for migrating users

[ryanfeeley]

So that I don't create a second Firefox Account, as a migrating user, I want to be notified about being able to migrate using an Firefox Account that has a different email address than the one I am migrating.

Full flow: https://github.com/mozilla/addons/blob/master/docs/ux/fxa-migration.rst

[shane-tomlinson]

Ref #2476, #2477

[vladikoff]

@rfk what do we do here?

[shane-tomlinson]

We should log an event for users who click "Sign in". How will this interact with #2778 (Allow users to sign in with the sign up screen).

[rfk]

For deciding whether or not to show the additional messaging, I see two options:

• Look at the client_id, and show it for some known list of reliers who are migrating their users from another auth system
• Have the relier send a query parameter like migration=true and show the messaging if that's present.

I think I slightly prefer the second option, and IIRC we used something similar for handing custom messaging during the sync1.1 migration.

[vladikoff]

from mtg: base this on sync migration

[shane-tomlinson]

Have the relier send a query parameter like migration=true and show the messaging if that's present.

In the AMO migration support spec, migration=amo is specified.

In order to trigger the additional migration indicator, AMO should include migration=amo as a query parameter when beginning the OAuth login flow.

migration=amo is better than migration=true because it allows us to show tailored migration text for different migrations w/o resorting to a client_id lookup.

Both the signin and signup pages currently show Sync migration text. Both screens will need to be updated to handle the new query param value. The Sync specific text on the signin page should only be shown for Sync migrations, i.e., where the migration query parameter is sync11 or sync15 (I'm unsure of sync15 is possible, but it shows up in our tests). On the signup page, the Sync text should be shown for migration is sync11, sync15, and the AMO text if migration is amo.

Ref #1906, #1928, #2786, #2823

[shane-tomlinson]

From this comment in bugzilla, it looks like the only possible migration value is sync11. The future proof thing to do is probably count all migration strings with the sync prefix as requiring the Sync specific migration text.