add sync beta message

[diracdeltas]

fix #8121

test plan:

1. go to about:preferences#sync
2. you should see text saying that sync is still being completed

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

[bbondy]

++