[Cashtab] Better label for show seed switch

Summary: The whole "i understand" thing does not make sense when the switch is on. hiding the label when it is on looks dumb. So, just simplify it.

Test Plan: proofread, npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15887