[Cashtab] Seed warning confirmation check

Summary:
T3329

Adds a conditional warning upon opening the seed phrase dropdown in Settings whereby the user needs to click the checkbox for the seed to be displayed.

Test Plan:
npm test
npm start
Navigate to Settings, open the seed phrase dropdown, click the checkbox and ensure the displayed seed corresponds to the active wallet
Ensure unchecking the checkbox hides the seed phrase
Repeat the above for a freshly created wallet
Repeat the test in extension mode

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14719