Sync promotion in Passwords and Bookmarks

[CDRussell]

Task/Issue URL: https://app.asana.com/0/72649045549333/1207997744789992/f

Description

Feature flag and data storage for sync promotions.

Steps to test this PR

• QA-optional for this branch of the stack

[CDRussell]

• #4921
• #4895
• #4892
• #4891 👈
• develop

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Merge activity

• Aug 29, 11:58 AM EDT: @CDRussell started a stack merge that includes this pull request via Graphite.
• Aug 29, 11:58 AM EDT: @CDRussell merged this pull request with Graphite.