Polish settings-sync UI

[miguelsolorio]

UX

• Sync: provide better progress information when sync is turning on Sync: provide better progress information when sync is turning on #100411
• Sync commands are undiscoverable Sync commands are undiscoverable #100425
• 🏃 UX: Improve first time sync user experience UX: Improve first time sync user experience #100346
• 🏃Synced data view is overwhelming Synced data view is overwhelming #100347
• Sync should allow you to replace synced data Sync should allow you to replace synced data #100791
• Use the same "sync" terminology across the product and docs Use the same "sync" terminology across the product and docs #100315
• Skip sync configure dialog when re-enabling sync Skip sync configure dialog when re-enabling sync #100336

Bugs

• Closing browser when Settings Sync starts leaves app in broken state Closing browser when Settings Sync starts leaves app in broken state #100314
• Turning on sync from an old device fails Turning on sync from an old device fails #100348
• Extensions list doesn't auto-update when sync is installing extensions Extensions list doesn't auto-update when sync is installing extensions #100415
• Synced data shows entries with no changes Synced data shows entries with no changes #100416
• Restoring extensions data from synced view shows wrong data Restoring extensions data from synced view shows wrong data #100448

cc @RMacfarlane @Tyriar

[sandy081]

Moving this to next milestone as most of the requests will be handled then.