[Cashtab] Prevent auto-capitalization of mnemonic seeds

Summary:
Users typing in mnemonic seeds on mobile devices experience auto-capitalization of the first word, which makes the seed invalid. This diff changes the input type to "email" to prevent this from happening.

Task T1687

Test Plan: npm test

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9855