fix(client): We broke password managers with `autocomplete=off` on password fields!

* Always add `autocomplete=off` when the relier is Sync.
* Add `autocomplete=off` to text fields acting as passwords, otherwise passwords are displayed in a form autocomplete dropdown. Yuck.

fixes #1788
Merge pull request #1989 from mozilla/issue-1788-autocomplete-off

fix(client): Only add `autocomplete=off` when in sync or to text fields acting as passwords.