feat: improve validation for OtpForm and LoginForm

[zxt-tzx]

While working on opengovsg/CheckWho#251, thought to transpose the code changes to checkfirst.

Advantages:

• More stringent frontend validation for email; must be @.gov.sg email, and not just any email
• Error case looks more aesthetic using react-hook-form and Chakra UI's FormControl, compared to default HTML

BEFORE:

AFTER:

[LoneRifle]

@tan-zx - given that we allow other quasi-government institutions to use CheckFirst (most notably the medical clusters), could you take the trouble to rework this PR so that we only take on board the OTP validation? Thanks =)